At least 34 people have been killed in the Syrian city of Hama after security forces opened fire on a demonstration against President Bashar al-Assad.

An activist said today was the bloodiest incident in the crackdown of the 11-week revolt against President Bashar al-Assad's rule.

In a pattern seen every Friday since mid-March, protestors have marched out of mosques after noon prayers, to be met by security forces intent on crushing a revolt.

Three residents said security forces and snipers fired at tens of thousands of demonstrators gathered in the city centre. This afternoon saw one of the biggest protests seen so far in Hama.

Protests in Hama have a particular resonance, since the city was attacked in 1982 by Assad's father, then President Hafez al-Assad, who crushed an armed Islamist uprising, killing up to 30,000 people and razing parts of the city to the ground.

Syrian forces also opened fire on demonstrations in the eastern city of Deir al-Zor and in Damascus' Barzeh district.

Activists and residents said thousands of people marched in the northwestern province of Idlib, the Kurdish northeast, several Damascus suburbs, the cities of Homs and Hama and the town of Madaya and Zabadani, in the west.

Analysts say protests continue to spread despite the military crackdown, but have shown no sign yet of reaching the scale needed to topple Assad's rule.

Rights groups say Syrian security forces have killed more than 1,000 civilians in the unrest, provoking international outrage at Assad's ruthless handling of the demonstrators and leading U.S. Secretary of State Hillary Clinton to say Assad's legitimacy 'had nearly run out.'