More than 60 people were killed in Syria as thousands of anti-government protesters took to the streets after Friday prayers, rights activists say.
At least 53 people are believed to have died when security forces fired on a crowd of about 50,000 people in the central city of Hama. Some accounts put the death toll at more than 100.
The opposition had dedicated the day to children killed during the uprising.
The army meanwhile continued its assault on the central town of Rastan.
Scores have been killed in the past few days as troops and tanks attempted to quell protests there. At least two civilians died on Friday, the Local Co-ordination Committees said.
"Since early this morning... there has been extensive bombardment with heavy and medium gunfire, using tanks, armoured vehicles and automatic machine gunfire," a Rastan resident told BBC Arabic on Friday.
"There was also random shelling, and there are snipers widespread on all the rooftops of the government buildings."
State television said about 80 security personnel had been wounded. Reports from Syria are hard to verify independently, as foreign journalists are not being allowed into the country.
On Thursday, US Secretary of State Hillary Clinton said President Bashar al-Assad's legitimacy had "nearly run out".
She said the US had done all it could to exert pressure on Syria, but she lamented the lack of a strong, united international response.
On Wednesday, Mr Assad launched a "national dialogue" and freed hundreds of political prisoners as part of a general amnesty.
Human rights groups say more than 1,100 people have been killed since pro-democracy demonstrations began in mid-March.
Organisers of the protests have tried to give each week of the uprising a special theme, and this latest day was dubbed the "Friday of the Children of Freedom", reports the BBC's Jim Muir in Beirut.
It comes at the end of a week when the UN Children's Fund (Unicef) highlighted the death of at least 30 young people so far.
The week has also seen a 13-year-old boy from the south, Hamza al-Khatib, emerge as the unofficial icon of the Syrian revolution. Activists said he had been tortured to death by security forces - something the authorities have denied.
'Blood was everywhere'
By all accounts, the Hama demonstration was one of the biggest in Syria so far with tens of thousands of people joining in, many of them streaming in from the surrounding countryside, our correspondent adds.
"This is a natural reaction to the increased killings and lack of seriousness by the regime for any national reconciliation," said Rami Abdul Rahman of the Syrian Observatory for Human Rights.
Security forces opened fire on the protesters in Hama's old quarter and the nearby Assi square, activists said, killing at least 53. Several hundred others were also wounded, they added.
"The firing began from rooftops on the demonstrators. I saw scores of people falling in Assi square and the streets and alleyways branching out. Blood was everywhere," one witness told the Reuters news agency.
"It looked to me as if hundreds of people have been injured but I was in a panic and wanted to find cover. Funerals for the martyrs have already started," he said.
State television said the protesters were "trying to clash" with the security forces and had attacked and set fire to public buildings, including the city courthouse. Armed men had used the presence of the crowds to carry out attacks, and three "saboteurs" had been killed, it added.
Hama was the scene of a brutal crackdown in 1982 ordered by Hafez al-Assad, the president's late father, which left at least 10,000 dead.
Thousands of people also reportedly demonstrated on Friday in and around the capital, Damascus, where there unrest has been limited.
Some 2,000 people marched in the northern district of Rukn al-Din, while police used batons to beat demonstrators in the southern district of Midan, the Syrian League for Human Rights told AFP.
There were also rallies reported in the Damascus suburbs of Barzeh, Jadaidat Artouz, Darayya and Zamalka.
Further demonstrations were said to have taken place in the city of Homs, the eastern town of Deir al-Zour, the western towns of Madaya and Zabadani, and the north-western town of Idlib.
Mr Abdul Rahman said troops killed one person in the village of Has, in Idlib province, while another rights activist, Mustafa Osso, told the Associated Press that security forces shot dead eight protesters Homs and three in Deir al-Zour.
In the southern city of Deraa, where protests first broke out, hundreds defied a military curfew and took to the streets chanting "No dialogue with killers", activists said.
There were also reports that the internet service had been cut in several parts of the country, apparently to prevent activists from uploading footage of the protests and repression. US firm Renesys confirmed the outage and said two-thirds of all Syrian networks were unavailable.