Local activists said most bodies were found in houses and basement shelters of residential buildings and appear to have been shot dead by troops who had stormed the premises.
“Assad’s army has committed a massacre in Daraya,” said Abu Kinan, an activist in Darya, using an alias to protect himself from reprisals.
“In the last hour, 122 bodies were discovered and it appears that two dozen died from sniper fire and the rest were summarily executed by gunshots from close range,” Abu Kinan told Reuters by telephone.
The activist said he witnessed the death of an 8-year-old girl, Asma Abu al-Laban, shot by army snipers while she was in a car with her parents.
“They were trying to flee the army raids. Three bullets hit her in the back and her parents brought her to a makeshift hospital. Nothing could be done for her,” he said.
The organization, which monitors Assad’s military crackdown, said 310 people were killed in Damascus and its environs, including Daraya, 40 in the northern province of Aleppo and 28 in Syria’s Sunni tribal heartland region of Deir al-Zor.
The rest were reportedly killed in the Idlib, Deraa, Hama and Homs, outlying provinces where poverty and discontent with Assad’s minority Alawite rule have been building up since bloody repression by Assad’s father, the late President Hafez al-Assad, killed tens of thousands of people in the 1980s.
Video footage from activists showed numerous bodies of young men side-by-side at the Abu Suleiman al-Darani mosque in Daraya, many with what looked like gunshot wounds to the head and chest.
“A massacre,” said the voice of the man who appeared to be taking the footage. “You are seeing the revenge of Assad’s forces ... more than 150 bodies on the floor of this mosque.”
The official state news agency said: “Our heroic armed forces cleansed Daraya from remnants of armed terrorist groups who committed crimes against the sons of the town and scared them and sabotaged and destroyed public and private property.”
The southern fringe of Damascus is a frontline in what has snowballed over the last 17 months from anti-Assad protests into a sectarian civil war.
Syrian forces had launched a deadly assault in the southwestern belt of Damascus on Saturday, in what activists said was a new bid to crush “once and for all” the insurgency in the capital.
Combat helicopters and tanks also pounded rebel-held areas of the battered northern city of Aleppo, an AFP journalist and monitors said, as the army pressed on with its war against fighters seeking to topple Assad.
The fresh violence 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, now in its 18th month.
Brahimi, who takes over from former U.N.-Arab League envoy Kofi Annan next month, held talks with U.N. leaders in New York on Friday, saying the Syrian people “will be our first masters.”
Annan, a former U.N. chief, quit earlier this month after the failure of his six-point plan to try to bring peace, which was left in tatters by the relentless bloodshed and divisions among world powers over how to tackle the conflict.