The Syrian military killed at least 200 people in a Damascus suburb Saturday, according to opposition activists, an attack that appeared to be part of a brutal campaign in recent days to root out rebel fighters and their supporters in the areas surrounding the capital.

The attack in Darayya, four miles southwest of Damascus, killed at least four children, according to activists, who said about 122 victims appeared to have been shot at close range. “Many of the victims were handcuffed and shot on the walls,” Abu Kinan, an activist in Darayya, said via Skype. “We can tell from the bodies we got that they were mass executed.”

The report of the killings came as the head of the U.N. observer team in Syria, Lt. Gen. Babacar Gaye, left the country, effectively ending a troubled mission that began last April. The team has carried out only minimal operations since June because of widespread violence and has been attacked at least once.

Earlier this month, the United Nations appointed Lakhdar Brahimi the new special envoy to Syria, with responsibility for mediating an end to the conflict — a prospect that seems increasingly unlikely as the combatants have ramped up attacks and their international supporters are at loggerheads.

Opposition activists estimate that 300 people may have been killed by the Syrian military in Darayya, which is close to al-Mazzeh military airport, in the past three days alone. The attacks on the suburb appear to have escalated since rebel fighters began disarming and withdrawing from the area to protect civilians, activists say.

Troops seemed to have gone through the suburb and systematically killed people Saturday. A video posted online Saturday showed several rows of corpses, which appear to be mostly men, wrapped in colorful blankets inside the Abu Suleiman al Durani mosque in Darayya. The person filming chants “God is Great” as he goes through the room.

Online videos of the government assault in recent days show multiple shells battering buildings and kicking up huge clouds of dust, along with graphic footage of rows of charred corpses lined up in a cemetery.

Though Darayya was one of the hardest-hit areas in the country Saturday, it was only one of approximately half a dozen cities where the Syrian military carried out operations. Several neighborhoods in the northern commercial hub of Aleppo were hammered by aerial bombing, and Daraa, in the south, was also shelled, according to the Local Coordination Committees network, which put the day’s death toll at 330 — one of the largest number of people killed in a single day since the conflict began in March 2011.

Separately Saturday, one of 11 Lebanese pilgrims taken hostage in Syria in late May, Hussein Ali Omar, was freed by his rebel captors; he flew home to Beirut. The abduction of the pilgrims, who are Shiite Muslims, had increased sectarian tensions in Lebanon, since many Shiites support the Syrian government while many Sunnis support the Syrian opposition.

Ali Omar, the hostage’s son, said his father had been released after a group of Muslim scholars negotiated to get him out. But it was a bittersweet moment when he was led away from his fellow captives. “My father was happy and sad at the same time,” he said. “Because his friends thought he would be executed, and he did not have the chance to let them know he was released.”

In an apparently unrelated development, a spokesman for the Lebanese Meqdad clan announced Saturday that the clan would be releasing a number of Syrian hostages. The Meqdads kidnapped the Syrians last week in an attempt to barter for the release of one of their clansmen being held in Syria.

Suzan Haidamous contributed to this report.