Appears to be a string of roadside bombs.
Via NY Post:
Syrian army troops on Wednesday killed 175 rebels, many of them al-Qaida-linked fighters, in an ambush described as one of the deadliest attacks by government forces against fighters near Damascus, according to state media.
An opposition group said the dawn ambush — part of a government effort to secure the capital — was carried out by the Lebanese Hezbollah group, which has been instrumental in helping President Bashar Assad’s regime push back rebels entrenched in the suburbs of the capital city.
Syrian state news agency SANA quoted a field commander in the eastern Ghouta area as saying most of the rebels killed in the assault near Oteibah lake southeast of Damascus belonged to the al-Qaeda-linked Nusra Front rebel group. The report said several of those killed were foreign fighters who came to Syria from Saudi Arabia, Chechnya and Qatar. […]
The agency posted several photographs on its website showing dozens of bodies of men, some with leg wounds, lying in a dirt track of an open field. Some were wearing fatigues, but most wore civilian clothes and appeared to have been carrying bags of clothes and bottles of water that were scattered on the ground, suggesting they were changing locations when they were ambushed.
The Britain-based Syrian Observatory for Human Rights said at least 152 rebels were killed, most of them Nusra Front and other fighters from Islamic brigades. “This is the heaviest loss for Nusra Front and Islamic brigades since the start of the revolution,” said Rami Abdurrahman, director of the rights group.
Reinforcing that report, Hezbollah’s Al-Manar TV aired exclusive footage of what it said was the ambush, showing at least two large bombs that were detonated along the route used by the opposition fighters.