Protesters in Syria have defied a security crackdown, turning out in thousands to attend the funerals of pro-democracy demonstrators and calling for the removal of President Bashar al-Assad, witnesses say.

Mourners at Nour Mosque in the central city of Homs on Sunday shouted "Leave, leave," at the funeral of six out of 11 people that rights groups said were killed by security services a day earlier.

 "The shooting was in cold blood. People were streaming peacefully out of the cemetery," a resident of Homs said.

Anti-government demonstrators also rallied in an eastern town after an activist immolated himself on Friday, echoing the self immolation of a Tunisian vegetable trader last year that sparked protests across the Arab world.

A human rights campaigner said 17-year-old activist Mohammad Akram al-Tumah set himself alight in the eastern town of Mayadeen, days after he was released from custody by state security agents.

"He set fire to himself in front of the state security building as a demonstration was taking place there demanding the release of political prisoners in the compound," the rights campaigner said, speaking on condition of anonymity.

"Protesters rushed to stop him but it was too late. Tumah died in hospital on Saturday," he said.

In Saqba, a witness told Reuters by phone that mourners also chanted the name of "Martyr Ziad al-Qadi", reportedly killed when security forces fired live rounds at a demonstration in the suburb of capital Damascus on Saturday.

Death toll

The Syrian Observatory for Human Rights, which is based in Britain, said it had the names of 863 civilians who had been killed in shootings by security forces since the pro-democracy uprising erupted 10 weeks ago.

Assad has largely dismissed the protests as part of a foreign-backed conspiracy to sow sectarian strife in Syria.

Syrian authorities blame most of the upheaval on "armed saboteur groups", backed by Islamists and foreign powers, who they say have killed more than 120 soldiers and police.

Syria has barred most international media since the protests broke out two months ago, making it difficult to verify accounts of the violence.

The unrest has posed a grave challenge to Assad's rule.

In response, he has lifted a 48-year state of emergency and issued a decree to grant citizenship to stateless Kurds. But he has also sent tanks to several cities to stamp out demonstrations, witnesses said.