Officials say the blast occured in a car parked in Beruit's Christian sector. It is unclear who was responsible.
The explosion occurred in the wake of fighting today between troops and Islamic militants in the northern part of the country, killing at least 39 people.
The fighting erupted with Fatah al-Islam in the city of Tripoli and the adjacent Palestinian refugee camp Nahr al-Bared. Militants also attacked an army patrol to the south.
Security officials say 22 soldiers and 17 militants were killed. There were also reports that at least six civilians were killed inside the camp.
By long-standing agreement, Lebanese police and soldiers do not enter Palestinian refugee camps.
Authorities say the fighting began when security forces tried to detain suspects in a bank robbery outside the refugee camp.
Information for this story is provided by AP, AFP, and Reuters.