The popular revolt against Syrian President Bashar Assad's regime has proved remarkably resilient, with protests erupting every week despite the near-certainty the government will respond with bullets and tear gas. The UN estimates the regime crackdown on the protests has killed 3,000 people since March.

Much of the bloodshed on Friday happened after the protests had ended and security forces armed with machine guns chased protesters and activists, according to opposition groups monitoring the demonstrations. Authorities disrupted telephone and Internet service, they said.

The Syrian opposition's two main activist groups, the British-based Syrian Observatory for Human Rights and the Local Coordinating Committees, gave figures for the protesters killed on Friday ranging from 29 to 37.

The flashpoints were Homs and Hama in central Syria, where opposition to the regime is strong. Hama is the site of a massacre nearly 30 years ago which has come to symbolise the ruthlessness of the Assad dynasty.

Rami Abdul-Rahman, the head of the observatory, said security forces in Homs were firing machine guns as they conducted raids in search of protesters and activists. In Hama, there were heavy clashes between the army and gunmen believed to be army defectors.

Syria has largely sealed off the country from foreign journalists and prevented independent reporting, making it difficult to confirm events on the ground. Key sources of information are amateur videos posted online, witness accounts and details gathered by activist groups.

Communications were spotty on Friday in the Damascus suburb of Douma and in Homs. The move appeared to be an attempt to cut off the opposition's ability to organise and report on the protests.

"There was a very fierce reaction to the protests in Homs today," said Syria-based activist Mustafa Osso. Syrian forces opened fire as some 2,000 people gathered for protests, he said.

"There are many injured as well. Hospitals are having a hard time coping with the casualties," Osso told The Associated Press.

Majd Amer, an activist in Homs said sporadic gunfire could be heard as protesters poured out of mosques following Friday prayers.

It is difficult to gauge the strength of the revolt in Syria, a country of 22 million people. The crackdown does not appear to have significantly reduced the number of protests, but neither does the regime appear to be in any imminent danger of collapse.

The regime appears to lack sufficient numbers of loyal troops to garrison all the centers of unrest at the same time, so government forces will often sweep through an area in the wake of protests, breaking up new gatherings and hunting activists, before being deployed elsewhere.

The result has been a monthslong stalemate. Still, the capture and subsequent death of Libya's Moammar Gadhafi, under still-unclear circumstances, has energized the opposition. Last week, thousands of Syrians took to the streets shouting that Assad will be next.

The protests come amid efforts by the Arab League to end the bloodshed, and debates within the opposition on how to bring international pressure to bear on the regime.

On Friday, many protesters said they wanted a no-fly zone established over Syria to protect civilians in case the Syrian regime considers attacking protesters from the sky, the activist groups said.

The protesters also called for international monitors, although most opposition groups reject the idea of foreign military intervention.

The Syrian government insists the unrest is being driven by terrorists and foreign extremists looking to stir up sectarian strife.