Three other smaller explosions also were reported in the capital, killing one person.
Meantime, thousands of Syrians protested elsewhere to denounce persistent violence by President Bashar Assad's regime.
Fridays have typically been a day of anti-government protests since an uprising against the regime began 13 months ago.
The violence was the latest blow to a peace plan brokered by special envoy Kofi Annan that called for a cease-fire to go into effect two weeks ago. The truce has been roundly ignored on the ground.
The Associated Press contributed to this report.