State television said the "terrorist car bomb" had killed five people and wounded 32.
Opposition activists said the bomb had gone off near a makeshift children's playground built for the Eid al-Adha holiday in the southern Daf al-Shok district of the capital.
Fighting erupted around Syria earlier as both sides violated the Eid al-Adha ceasefire arranged by international peace envoy Lakhdar Brahimi. Violence was far less intense than usual, with only seven killed before the car bombing, according to the Syrian Observatory for Human Rights, far below the 140 it reported on Thursday.
The Syrian military had said it would hold fire for four days following Mr Brahimi's ceasefire appeal The UN-Arab League envoy had hoped to build on the truce to calm a 19-month-old conflict against president Bashar al Assad that has killed an estimated 32,000 people.
However, Syrians' hopes of celebrating Eid al-Adha, the climax of the Haj pilgrimage to Mecca, in peace were short-lived.
"We are not celebrating Eid here," said a woman in a besieged Syrian town near the Turkish border. "No-one is in the mood to celebrate. Everyone is just glad they are alive."