About 320 bodies, including women and children, were found in houses and basements in the town of Daraya, southwest of Damascus, according to activists who said on Sunday most had been killed "execution-style" by troops.
Activists uploaded several videos to the Internet showing rows of bloodied bodies wrapped in sheets. Most of the dead appeared to be young men of fighting age, but at least one video showed several children who appeared to have been shot in the head. The body of one toddler was soaked in blood.
Due to restrictions on non-state media in Syria, it was impossible to verify the accounts independently.
Clashes are raging across Syria as the 17-month-old rebellion grows increasingly bloody, particularly in the northern city of Aleppo, where the army and rebels appear stuck in a war of attrition.
Fighting in Aleppo on Sunday was the heaviest in the past week, according to Reuters journalists on the ground.
Fighter jets dropped bombs and fired missiles on rebel-held districts in the south of Aleppo, Syria's largest city, as residents fled in panic. Reuters journalists there heard heavy explosions as clouds of black smoke rose a mile into the air.
Rebels say they control at least half the city of 2.5 million, but their hold is fragile as long as Assad's forces can unleash their air power against fighters who are comparatively lightly armed.
The uprising, which began as peaceful protests, has become a brutal civil war. United Nations investigators have accused both sides of war crimes but laid more blame on government troops and pro-government militia than on the rebels.
The killings in Daraya, a working class Sunni Muslim town that sustained three days of bombardment before being overrun by the army on Friday, raised the daily death toll to 440 people on Saturday, one of the highest since the uprising began, an activist network called the Local Coordination Committees said.
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."
The death toll for the following day, Sunday, was more than 90, including civilians and fighters, according to another activist network, the Syrian Observatory for Human Rights.