Syria's security forces on Friday shot dead two civilians in the flashpoint central city of Homs, where anti-regime protests were expected after the main weekly Muslim prayers, rights activists said.

The killings took place at a security force checkpoint in the city's Bab al-Sibaa neighbourhood, the Syrian Observatory for Human Rights said in an emailed statement to AFP in Nicosia.

Syria's opposition movement, boosted by the killing of Libya's ousted dictator Moamer Kadhafi, called for fresh protests across the country in support for the Libyan people and against the regime of President Bashar al-Assad.

The Friday prayers have become a lightning rod for protest against Assad, with thousands pouring out of mosques across the country each week to join demonstrations despite the strong likelihood of brutal action by the security forces.

Assad has been faced with a popular revolt since March 15, which has been harshly repressed, resulting in the deaths of more than 3,000 people, mostly civilians, according to the United Nations.

"Your turn has come Doctor (Assad)," protesters wrote on their Facebook page, Syrian Revolution 2011.

"Give whatever delay you want, our revolution will vanquish, we will continue to call with all our voice to bring down the regime and to tell the world that the Syrian people will now surrender," it added.

According to the Britain-based Observatory, at least five civilians were shot dead by security forces in various parts of the country on Thursday.

Violence in Syria has intensified in recent weeks as defections from the army increase, sparking armed confrontations amid UN warnings of the potential for full-blown civil war in the Arab state.