Six people were reported killed as hundreds of thousands of protesters took to the streets in cities around Syria.
It was one of the biggest series of demonstrations against the regime of President Bashar Assad since the uprisings began more than three months ago.
Syrian rights groups say more than 1,400 people have been killed, most of them unarmed protesters, since mid-March.
The regime disputes the toll, blaming “armed thugs” and foreign conspirators for the unrest that has posed the most serious challenge to the Assad family’s 40-year ruling dynasty in Syria.
Syria-based rights activist Mustafa Osso said huge protest crowds moved into the streets after noon prayers in places across the country, including the capital Damascus.
Among the largest rallies was in the central city of Hama, where about 200,000 people went out in the streets, said Omar Idilbi, a spokesman for the Local Co-ordination Committees, which track the protests in Syria.
Idilbi said security forces killed at least three people in another central city, Homs, after opening fire.
In separate clashes, three people were killed during a military operation seeking to choke off the flow of refugees heading across the border to Turkey, said Rami Abdul-Rahman, the London-based director of the Syrian Observatory for Human Rights.
More than 10,000 Syrians have already taken shelter in refugee camps in Turkey to escape the violence.