Thousands of Syrians took to the streets to rally against President Bashar al-Assad on the first Friday of Ramadan in support of the embattled protest hub of Hama, rights activists said.

Communications were completely cut off as the army stepped up an operation to crush dissent in Hama, the central city where security forces killed at least 30 civilians and wounded dozens more earlier in the week.

"Thousands of demonstrators marched in Deir Ezzor, Daraa and Qamishli in support of the city of Hama despite the extreme heat," Abdel Karim Rihawi of the Syrian League for the Defence of Human Rights told AFP in Nicosia by phone.

Rami Abdel Rahman, head of the British-based Syrian Observatory for Human Rights, said "more than 12,000 people marched in Bench" in Idlib province "to demand the fall of the regime and express their support for Hama and Deir Ezzor."

"Hundreds of people came out of the Al-Mans Uri mosque in Jablah, chanting 'God is with us'," he told AFP.

The call for Friday's protests came from activists on Facebook group The Syrian Revolution 2011, a driving force behind the demonstrations calling for greater freedoms since mid-March.

The Assad regime has sought to crush the democracy movement with brutal force, killing more than 1,600 civilians and arresting thousands of dissenters, rights activists say.

Its latest crackdown has centred on Hama, where at least 30 people were killed on Wednesday by tanks shelling the city centre.

The city was isolated on Friday, and the military continued an operation to combat what Assad's regime calls "armed terrorist gangs" responsible for the deadly unrest.

State media reported that army units were removing "roadblocks set up by terrorist groups that have blocked roads and damaged public and private property, including police stations, using various weapons."