Dozens of people were wounded in the explosion in the town of Suq Wadi Barada, said the Britain-based Syrian Observatory for Human Rights, indicating that the death toll could rise.
The town is under rebel control, but troops loyal to President Bashar al-Assad were positioned just outside it, it said.
State news agency SANA said "the car exploded while the terrorists were packing it with explosives near the Osama Bin Zeid mosque. Terrorists and civilians were killed."
The outskirts of the capital have seen fierce fighting in recent days as Assad's troops have sought to tighten the noose around rebel areas under siege for months.
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Yesterday Syrian troops captured a town southeast of Damascus, which a military source described as an "important centre for terrorists," referring to the rebels who have been battling the regime since 2011.
The operation was part of a larger effort to close in on Eastern Ghouta, a ring of suburbs besieged by the army for months, which was targeted in an August chemical attack that killed hundreds of people.
The Observatory today described a similarly dire situation in the central city of Homs, where it said some 3,000 civilians are trapped in an area sealed off by regime forces for more than a year.
"Three thousands civilians, among them 500 aged over 70, are living exclusively off the little food that had been stored in the besieged districts of Homs," said Observatory director Rami Abdel Rahman.
Further north, in the Sfeirah district near Aleppo, some 130,000 Syrians have fled non-stop heavy bombing in a "massive exodus" this month , according to Doctors Without Borders (MSF), which has staff on the ground.