A blast rocked a densely populated neighborhood of south Beirut Thursday, killing at least five people in an area known as a Hezbollah stronghold.
The explosion wounded about 20 people. Hezbollah's Al-Manar television showed ambulances racing to the scene and burning wreckage.
The nature of the blast is still unclear, but Lebanese Health Minister Ali Hassan Khalil told reporters the government was fighting a "battle against terrorism."
Television footage showed a charred automobile covered with leaves, and rubble falling from balconies of nearby buildings.
Later Thursday, the United Nations Secretary-General Ban Ki-moon and the U.S. State Department condemned the attack. The United States also called on all parties to "refrain from retaliatory acts."
The explosion comes less than a week after a massive car bombing in central Beirut killed former Lebanese finance minister Mohamad Chatah, a harsh critic of Syrian President Bashar al-Assad.
Five other people were killed and about 70 wounded in that blast.