Meanwhile, Russia and Turkey dismissed growing calls led by US President Barack Obama for Assad to quit, offering the embattled Syrian leader rare support despite a damning UN report on his "apparent shoot to kill" policy.
On the political front, a group of "revolutionary blocs" formed a coalition vowing to bring down the regime and paid tribute to more than 2,000 civilians killed in a crackdown on protesters since mid-March.
The Syrian Observatory for Human Rights said 15 people were killed, including an 11-year-old and a 72-year-old, in the southern province of Daraa, epicentre of the anti-regime protests that erupted March 15.
Six were killed in the central city of Homs, one in the Harasta suburb of Damascus and another in the capital's suburb of Douma.
The Observatory said security forces opened fire on protesters, also wounding 25 people, in the Ghabagheb, Inkhil, Al-Herak and Nawa neighbourhoods in Daraa, but the official SANA news agency blamed the shooting on "armed men." SANA said a policeman and a civilian were killed in Ghabagheb and six security forces wounded.
People poured into the streets of major towns as they emerged from the weekly prayers, with the largest anti-regime demonstration reported in Homs.