Syrian security forces have used tear gas and batons to disperse tens of thousands of protesters in the capital, Damascus, witnesses said.
The protesters called for reforms, while some demanded the overthrow of President Bashar al-Assad.
The protests, in Damascus and other cities, are believed to be the largest in a month of unrest in which about 200 people have been reported killed.
Mr Assad has made some concessions while cracking down on dissent.
Thousands of people were reported to have demonstrated in a number of other Syrian cities, including Deraa, Latakia, Baniyas and Qamishli - places where violence has been previously reported.
State media reported that "small demonstrations" had taken place in different parts of the country and security forces did not intervene. Yellow card warning
The mass protest in the suburbs of Damascus marks a major escalation of Syria's month of unrest, which has largely bypassed the capital.
It is time for the Syrian government to stop repressing their citizens and start responding to their aspirations”
End QuoteHillary ClintonUS Secretary of State
Analysts said Friday's protests were the largest since they began in the southern city of Deraa on 15 March.
The unrest is seen as the biggest challenge to Mr Assad, who inherited power from his father in 2000.
The protesters say they want greater freedoms, including a repeal of the decades-old security law, which bans public gatherings of more than five people.
Some are calling for the overthrow of the president, who rules with a tight grip through his family and the security forces.
Mr Assad has offered some concessions, forming a new government on Thursday and pronouncing amnesty for an undisclosed number of people detained in the last month.
He has also sacked some local officials and granted Syrian citizenship to thousands of the country's Kurdish minority - satisfying a long-held demand.
The demonstrators in Damascus held up yellow cards, in a football-style warning to President Assad, AP news agency said.
"This is our first warning, next time we will come with the red cards," one protester said. 'Legitimate demands'
Other witnesses said the demonstrators tore down posters of Mr Assad they passed along their route and called for the overthrow of the president.
Reuters quoted a witness who said 15 busloads of secret police had chased people into alleyways north of the city's main Abbasside Square.
The United Nations and a number of Western governments have decried President Assad's use of force to try to quash the protests.
US Secretary of State Hillary Clinton called on Syria's authorities to stop using violence against their own people.
"The Syrian government has not addressed the legitimate demands of the Syrian people," she said after a Nato meeting in Berlin.
"It is time for the Syrian government to stop repressing their citizens and start responding to their aspirations."
Human rights campaigners say hundreds of people across Syria have been arrested, including opposition figures, bloggers and activists.
Mr Assad blames the violence in recent weeks on armed gangs rather than reform-seekers and has vowed to put down further unrest.
US officials have said Iran is helping Syria to crack down on the protests, a charge both Tehran and Damascus have denied.
Algerian President Abdelaziz Bouteflika, who has been in power for 12 years, has promised to amend the constitution to "strengthen democracy".
Delivering a long-awaited speech on state TV, he said a constitutional commission would be created to draw up the necessary amendments.
It was his first address to the nation in three months.
Unrest broke out in January with strikes, marches and rioting echoing protests elsewhere in the Arab world.
Under pressure to amend the constitution and limit presidential terms, the 74-year-old leader lifted the country's state of emergency in February, after 19 years.
However, the trigger for the unrest appears to be mainly economic, especially sharp increases in the price of food. New laws
Algerians gathered around TV screens to watch the president, who looked frail and spoke in a hoarse voice.
"To crown the institutional edifice with the aim of strengthening democracy, it is important to introduce the necessary amendments to the constitution," he said.
"Active political currents" would join constitutional law experts in drawing up the amendments, he said.
In addition, the electoral law would be "revised in depth", with the participation of "the political parties, whether present [in parliament] or not".
A new information law would be introduced to replace existing legislation under which journalists have been jailed after writing articles critical of the president.
The new law would include "regulations for responsible blogging", the president said.
The current constitution, adopted in 1996, was introduced to strengthen presidential law and ban religion-based parties following Algeria's murderous war between the military and Islamist militants, in which some 150,000 people were killed.
Political analyst Mohamed Lagab cautiously welcomed Mr Bouteflika's speech.
"Acknowledging that political reforms - and not only social and economic reforms - will help to solve the crisis is key," the Algiers University academic told Reuters news agency.
"But Bouteflika did not give a deadline for the reforms he announced and he did not sack the government yet," he added.
Abdullah, a doctor in Misrata, told the BBC he had seen evidence of the use of cluster bombs
Pro-government forces in Libya have been accused by a human rights campaign group of using cluster bombs, which are banned by more than 100 countries.
One of its photographers saw three mortar-launched projectiles explode over a residential area of Misrata, in western Libya.
A government spokesman denied the allegation, calling for more evidence.
New attacks by government troops have been reported in Misrata, the only western city still in rebel hands.
One resident, Aous, told the BBC he had seen indiscriminate bombing and shooting from 0630 (0330 GMT) on Saturday.
"I listened to bombing and shooting everywhere. And I saw smoke going up just a few metres from my house.
"The shooting was random in this area - there are families here. They didn't make a difference between any houses, just shooting."
As well as cluster munitions there have been a number of reports that Libyan forces are using the Soviet-designed Grad rocket system in their bombardment of Misrata.
The Grad, which launches multiple rockets from mobile launchers, has been blamed for a number of civilian deaths in recent days, including eight people in a bread queue. Reuters news agency reported large numbers of Grads fired into Misrata on Saturday.
Unexploded cluster bomblets are often brightly coloured and picked up by children
The BBC's Jon Leyne, in the eastern rebel stronghold of Benghazi, says the Grad is another indiscriminate weapon that - while not illegal - could be even deadlier than cluster bombs.
Launched from a truck containing up to 40 rocket pods and able to fire some 40 rockets in quick succession, Grads blanket a wide area and are said to have been used against the port of Misrata, a key link to the outside world for those in the city.
This is urban warfare of the nastiest kind, our correspondent says: on the one hand you have the rebels fighting the government and on the other government forces appear to be randomly shelling and using sniper fire in civilian areas.
There were also reports of renewed fighting near the eastern town of Ajdabiya, where rebels are regrouping on the road to the oil town of Brega. 'Appalling' Releasing photographs of cluster munitions, New York-based Human Rights Watch said three projectiles had exploded over Misrata's el-Shawahda neighbourhood on Thursday night.
The latest revelations and the continuing suffering in Misrata are a reminder that the Nato mission - to protect civilians - is not working sufficiently.
The Western countries leading the intervention have some thinking to do. The UK, France and the US have said very clearly that they want Col Gaddafi out, but they are going to have to work out how to make that happen.
They may want to go the UN Security Council, but it is inconceivable at the moment that they would get a stronger mandate calling for regime change and allowing them to use their ground troops, even in a training capacity. Resolution 1973 allows the air operation but it very specifically forbids a foreign military occupation. There have have been some discussions about other ways of using troops, perhaps to safeguard humanitarian convoys, perhaps the sea routes into Misrata. But that would take some discussion.
First discovered by a New York Times reporter and inspected by HRW researchers, the object photographed is said to be an MAT-120 120mm mortar projectile, which opens in mid-air and releases 21 "bomblets" over a wide area.
"Upon exploding on contact with an object, each submunition disintegrates into high-velocity fragments to attack people and releases a slug of molten metal to penetrate armoured vehicles," HRW noted.
HRW said the projectile it had examined had been manufactured in Spain in 2007, one year before Madrid signed the international Convention on Cluster Munitions.
Steve Goose, HRW's arms division director, said it was "appalling" that Libya was using such weapons, especially in a residential area.
"They pose a huge risk to civilians, both during attacks because of their indiscriminate nature and afterward because of the still-dangerous unexploded duds scattered about," he added.
HRW said it could not determine whether any civilians had been hurt by the cluster bombs which "appear to have landed about 300 metres [yards] from Misrata hospital".
The international Convention on Cluster Munitions, adopted in Dublin in 2008, prohibits its 108 signatories from using cluster weapons because of the threat they pose to civilians.
Libya is one of the states which has not signed the convention, along with countries such as the US, Israel, Russia and China.
The cluster bomb is dropped from a plane or fired from a rocket launcher.
Models differ but some can contain up to 200 bomblets
The canister starts to spin and opens while in mid-air, spraying the bomblets across a wide area
Each bomblet is about the size of a drink can and contains hundreds of metal pieces. When it explodes, it can cause deadly injuries up to 25m (80ft) away
Libyan government spokesman Moussa Ibrahim denied cluster bombs had been used in Misrata.
"I challenge them to prove it," he told reporters in the capital Tripoli.
Referring to inspections by humanitarian groups, he said: "To use these bombs, the evidence would remain for days and weeks, and we know the international community is coming en masse to our country soon. So we can't do this, we can't do anything that would incriminate us even if we were criminals."
There was no immediate comment from Spain, a signatory to the cluster munition convention, on the provenance of the bombs. No reinforcements
Rebels in Misrata have been holding out against attacks for two months and UK Foreign Secretary William Hague has stressed that Nato needs to act swiftly to prevent a "massacre" in the city.
He said on Friday that Nato had been constrained by the need to avoid civilian casualties but had probably prevented the city from being overrun by Col Gaddafi's forces.
On Friday a meeting of Nato foreign ministers in Berlin ended without a commitment from non-participating nations to contribute combat aircraft to the alliance's military operations over Libya.
The US, UK and France said in a joint statement that the threat to Libyan civilians will not disappear while Colonel Muammar Gaddafi remains in power.
However, Russia has suggested Nato has been exceeding its UN Security Council mandate to protect civilians.
The Afghan National Army is a frequent target for insurgent attacks
Five foreign and five Afghan troops have died in an attack in eastern Afghanistan, officials say.
A Taliban suicide bomber wearing a military uniform hit an Afghan army base near the city of Jalalabad, the Afghan defence ministry said.
Coalition officials said five foreign troops died but gave no more details. Four Afghan soldiers and four translators were said to be injured.
The attack was one of the deadliest in months against foreign troops.
It took place shortly after 0730 (0330 GMT) when the bomber approached the gate of the military base and detonated his explosives.
The Taliban claimed responsibility for the bombing, adding that the attacker was a "sleeper agent" who had served in the army for at least one month before launching his attack.
However, Afghan officials strongly denied that suggestion, insisting that the bomber was wearing a military uniform but not a serving soldier.
Saturday's attack was the third in the past 72 hours.
The earlier two attacks had claimed the lives of an influential tribal elder, Malik Zarin, and the police chief of Kandahar, Khan Mohammad Mujahid.
These attacks are an attempt by the insurgents to undermine the authority of the Afghan national police and army - the two institutions which are supposed to assume the responsibility of the country's security once the foreign forces return.
The insurgents are also trying to convey the message to the people, through such acts, not to send their family members to serve the police and army.
Coalition officials in Afghanistan confirmed that foreign troops had died on Saturday but did not specify the nationalities of those killed.
"Five International Security Assistance Force (Isaf) service members died following an insurgent attack in eastern Afghanistan today," a statement said.
The attack was the third in recent days blamed on Taliban rebels.
The first killed a tribal elder closely connected to Afghan President Hamid Karzai, while on Friday the police chief of the southern province of Kandahar was killed.
Khan Mohammad Mujahid - a former mujahedeen fighter - had survived two previous attempts on his life, one on his way home and another in his motorcade.
In the two most recent incidents, the attacker was wearing an offical Afghan uniform.
Despite a sales ban put in place to reduce the number of attacks, Afghan army and police uniforms are widely available throughout the country
