He said the Mazraa bombing “was similar to an earthquake — my house’s windows were broken.”

The Syrian Observatory for Human Rights, an anti-Assad group based in Britain that has a network of contacts in Syria, reported that at least 59 people had been killed by the Mazraa bomb, which the group described as a booby-trapped car next to a military checkpoint. It said that at least 16 of the dead were members of the security forces.

At least 13 other people in Damascus were killed, 10 of them in the security forces, in two other car bombings near checkpoints in the Barzeh district, in the northeast part of the capital, the group said.

Syrian rebels have been entrenched for months in suburbs south and east of Damascus, but they have been unable to push far into the center, although they have hit the area before with occasional mortar shells and increasingly frequent car bombs.

Such indiscriminate attacks have risked killing passers-by, exposing the rebels to accusations that they are careless with civilian life and property. Many Damascus residents have remained undecided in the civil war and fear that their ancient city will be ravaged like Aleppo and other urban centers to the north. At the same time, the government has decimated pro-rebel suburbs with airstrikes and artillery, leaving vast areas depopulated and traumatized.

Some outside experts speculated that the bombings on Thursday had been carried out by the more militant extremists among the rebels to weaken the government’s argument that it offers security, at least to the middle- and upper-income neighborhoods of Damascus.

“The opposition will have to go in and mess that up if they want people to leave the regime,” said Joshua M. Landis, the director of the Center for Middle East Studies at the University of Oklahoma and the author of the blog Syria Comment, which has tracked the conflict. “Messing up Damascus and forcing the city people to take sides is the name of the game.”