Responding to a call under the slogan "Your silence is killing us," the protesters emerged from mosques in cities around the country, rights activists said.
At least 50,000 people turned out in the eastern city of Deir Ezzor to honour four people killed there, and an estimated 300,000 attended the funerals of three of them, said Sami Abdel Rahman, of the Syrian Observatory for Human Rights.
Security forces shot dead one young man as protesters there were returning to their homes, Abdel Rahman said.
In Latakia, he said "a young man was killed and several others injured by gunfire from security forces who tried to disperse hundreds of demonstrators gathered at a square."
Another young man was killed by security forces in similar circumstances in Daraa, where security forces had also reportedly deployed in large numbers, said Abdel Karim Rihawi, who heads the Syrian League for the Defence of Human Rights.
And a fourth youth was killed at a checkpoint in the Damascus suburb of Kser, Abdel Rahman said. Abdel Rahman said more that 500,000 people protested in Hama, but that there were no reports of violence there.
However, in the central city of Homs, security forces "fired on demonstrators in the Karam al-Shami district, wounding several people."