Security forces have been on high alert since a suicide bomber killed one person and wounded 37 near the Syrian border on Friday in an attack that narrowly missed Major General Abbas Ibrahim, head of Lebanon's General Security department.
One security source said before the late-night explosion on Monday that security forces were hunting for two potential suicide bombers in the Lebanese capital.
Lebanon has suffered a wave of sectarian violence linked to the war in Syria which, like its neighbour Iraq, is fighting a Sunni insurgency. Militants, including the Islamic State in Iraq and the Levant (ISIL), are sweeping through Iraq towards the Shi'ite-led government in Baghdad.
"What is happening in Iraq isn't far from what is happening in Lebanon, but Lebanon will not let ISIL spread here," lawmaker Ali Ammar, from the Shi'ite militant and political group Hezbollah, told Al-Manar television.
Monday's explosion killed the bomber and wounded 19 people, Lebanon's civil defence force said.
An emergency worker at the nearby Sahel hospital said it treated 11 slightly-wounded people.
Two security sources said yesterday that a member of the security services had also been killed when the Mercedes car packed with explosives detonated.
Lebanon's state-run news agency identified him as Abdul Kareem Hodrej, an officer in Lebanon's General Security forces.