BEIRUT, Lebanon — A series of car bomb blasts shook the Syrian capital, Damascus, early Sunday, killing 21 people and highlighting security gaps in parts of the country controlled by President Bashar al-Assad, according to an opposition monitoring group.

A suicide bomber blew up the car he was driving near the heart of Damascus after being surrounded by security forces, while two other car bombs were detonated after security forces tried to intercept the vehicles on the edge of the city.

Syrian state television reported that eight people had died and 13 had been injured, but the Syrian Observatory for Human Rights, an opposition monitoring group based in Britain, reported that 21 were killed, including the three attackers.