in a mosque complex in Daraya after a massive offensive by troops battling to crush insurgents who have regrouped in the outskirts of the capital.
At least 320 corpses were found on Saturday and Sunday, the victims of a five-day onslaught by the army on rebel fighters in Daraya, the Syrian Observatory for Human Rights said.
The Local Coordination Committees, a network of activists on the ground in Syria, described it as a "massacre" by the regime of President Bashar al-Assad and said people in Daraya had been summarily executed and their bodies burnt.
Human rights groups have accused the regime of committing many atrocities since the uprising against Assad's government first erupted in March last year, and a UN panel said earlier this month it was guilty of crimes against humanity.
Militants issued a video on YouTube entitled "Massacre at the Abu Sleiman Addarani Mosque in Daraya" showing dozens of bodies lined up in dimly lit rooms.
"An odious massacre committed by the gangs of the Assad regime in the Abu Sleiman Addarani Mosque. More than 150 innocent martyrs fell in a brutal campaign by the criminal gangs against the city," said the commentary with the video, whose authenticity could not be verified.
State television said Daraya, a mainly Sunni Muslim town of some 200,000 people, was being "purified of terrorist remnants."
The army claimed to have retaken most of Damascus in late July, after about two weeks of intense fighting across the capital's southern belt. Most rebel Free Syrian Army fighters were forced out into the nearby countryside, but have since resumed hit-and-run operations, according to activists.
"Our valiant armed forces cleared the town of Daraya of the remnants of armed terrorist groups which committed crimes that traumatised the citizens of the town and destroyed public and private property," the government newspaper Ath-Thawra said on Sunday.
Activists had described the Daraya offensive as a bid to crush "once and for all" the insurgency in the capital after rebel fighters regrouped to the southern outskirts following an army offensive in the heart of the city last month.
"The criminal regime imposed a blockade and cut off necessary supplies to the town, then subjected it to indiscriminate shelling with heavy weapons and warplanes," the LCC said in a statement.
"Afterwards the gangs of killers entered the town and carried out summary executions, before dismembering and setting fire to the bodies."
Reports by activists cannot be independently confirmed because of severe restrictions on media operating in Syria.
At least 183 people were killed nationwide on yesterday, the Observatory said, as the brutal conflict that has convulsed Syria for 17 months showed no signs of abating in the face of deep divisions among world powers.
A report by UN investigators said on August 15 that government forces and their militia allies have committed crimes against humanity and that rebels have also carried out war crimes, although on a lesser scale.
In particular, it held government forces responsible for a massacre in the central town of Houla in May when 108 civilians, including 49 children, were killed in an atrocity that shocked the world.
Saturday's bloodletting erupted a day after new international envoy Lakhdar Brahimi admitted he was "scared" of the enormity of the task he faces to try to end the increasingly ferocious conflict.
Brahimi, who takes over from former UN-Arab League envoy Kofi Annan next month, held talks with UN leaders in New York on Friday.