(CNN) -- Syrian government forces killed scores of residents of a town south of the capital in an operation that began several days ago and ended Wednesday, an opposition activist and witness told CNN.
In all, 107 residents of Al Thiabieh were killed, said Abu Jaafr, the nom de guerre for a member of the opposition Thiabieh Coordination Committee, speaking by Skype from inside the town of about 70,000 residents, which is 17 kilometers (11 miles) from Damascus.
Abu Jaafr said he learned of the extent of the carnage, described as a "massacre" by one rebel group, only after going house to house to help retrieve bodies and provide what limited assistance they could to survivors.
The attack began Sunday, when regime forces began shelling the town, according to the AlJolan Media Center, an opposition media group in Al Thiabieh.
When Free Syrian Army rebel fighters from Al Thiabieh and nearby suburbs launched counterattacks, the military's shelling intensified; at one point, dozens of shells were raining down on the town at any given moment, Abu Jaafr said.
By Wednesday morning, unable to counter the heavy artillery attacks with their light weapons and small numbers, the FSA soldiers retreated, he said.
As they withdrew, the FSA soldiers -- two of whom had been killed in the onslaught -- called on residents to leave, but many refused, including Abu Jaafr. "We have always resisted and never bowed down, and people are too proud to surrender now," he said.
Within a half hour of the FSA's withdrawal, forces loyal to Syrian President Bashar al-Assad entered the town and set about raiding homes, killing about 50 residents -- most of them men, Abu Jaafr said.
A few hours later, al-Assad's troops withdrew, leaving the town decimated, added Abu Jaafr, who then joined other rebels in surveying the damage.
"In every home we entered, there was a dead body," he said. "The scream of the women and children pierced us and the men kept beating their chests and yelling in pain -- completely overcome with emotion. It was impossible for us; we did not want to move forward to the next house because we knew the scene would repeat again and again."
Many of the victims, who included two women and three children, had been targeted by snipers; 25 buildings had been leveled, he said.
By late Wednesday, the town was eerily quiet, he said. "There is nothing left here but crumbled buildings and hollow streets."
Also left were bodies. A YouTube video showed the bodies of more than a dozen men lying lifeless on the ground, many of their clothes stained deep red. Several appeared to have been shot in the head.
Most of Al Thiabieh's residents are Sunni. The town is near the Damascus suburb of Saida Zaynab, which is home to many of Syria's Shiite minority and home to a number of Hezbollah members. "The regime attacked us because they were afraid we would do something to Saida Zaynab," Abu Jaafr said.
Abu Jaafr's account of what he says happened in Al Thiabieh was corroborated by the AlJolan Media Center, an opposition media group as well as by the Local Coordination Committees of Syria, another opposition group, which described the killings as a "massacre."
CNN's Salma Abdelaziz contributed to this report