DAMASCUS – Syrian security forces opened fire on a massive sit-in demonstration in the central city of Homs overnight, sending thousands of protesters scattering, rights activists said Tuesday.

"The sit-in was dispersed with force. There was heavy gunfire," an activist reached by telephone in Damascus said, without being able to give details of possible casualties.

Other Syrian activists living in exile confirmed the reports but said they had not been able to get further details due to telephone communications being cut in Homs.

Activists had said more than 20,000 demonstrators on Monday occupied the main square of Homs, some erecting tents, a day after 11 people were killed by security forces in the industrial city and a nearby town during a day of massive nationwide protest.

Inspired by popular uprisings which toppled hardline rulers in Tunisia and Egypt, the protesters had vowed not to leave Al-Saa Square in the centre of Homs until the regime of President Bashar al-Assad fell.

They dismissed as insufficient a weekend pledge by Assad that he would lift nearly five decades of draconian emergency law and demanded the release of all political prisoners and an end to arbitrary arrests.

The interior ministry issued a warning late Monday that it would suppress an "armed revolt" undermining security in the country.

"The latest incidents have shown that... armed Salafist groups, particularly in the cities of Homs and Banias, have openly called for armed revolt," said a ministry statement carried by the official SANA news agency.

It accused such groups of killing soldiers, policemen and civilians, and of attacking public and private property, and warned that "their terrorist activities will not be tolerated."

The authorities "will act with determination to impose security and stability in the country" and will "pursue the terrorists wherever they are in order to bring them to justice and end the armed revolt", it said.

The ministry called on people to "name the places where the terrorists and suspects are and not to allow them to live among them".

Foreign Minister Walid Muallem said Syria will proceed with reforms as promised, but warned against "sabotage" by protesters, SANA reported.

At least 200 people have been killed by security forces or plain-clothes police since the start of the protest movement, according to Amnesty International.

Meanwhile the United States acknowledged supporting non-government groups and civil society to bolster democracy in Syria but declined to comment directly on a WikiLeaks report it was secretly financing opposition groups.