Former finance minister Mohammed Chatah died when a car bomb exploded outside government offices in the city centre of the Lebanese capital of Beirut.
Chatah was a senior aide to former Lebanese prime minister Saad Hariri. His driver and five other people were also killed and more than 70 were wounded, the National News Agency said, although later reports put the death toll at six.
The blast was heard across the city early on Friday morning. A plume of black smoke was seen rising in the downtown business and hotel district.
Several cars caught fire and police cordoned off the area.
"We ran like madmen. We saw people lying on the street. We saw mangled cars. We saw broken glass and twisted metal scattered everywhere," Firas al-Sheikh, a waiter at a nearby cafe, told the local Daily Star newspaper.
Chatah, 62, served as Lebanese ambassador to the US and was one of the closest aides to Hariri's father, former PM Rafik Hariri, who was killed in a truck bombing in Beirut in 2005.
Chatah was named finance minister when Hariri took the reins of the government in 2008. After Hariri lost the premiership in 2011, Chatah served as his senior adviser.
Hariri, a Sunni Muslim, heads Lebanon's main, Western-backed coalition, which is at odds with the Shia militant Hezbollah group.
The Syrian civil war has polarised Sunni and Shia communities across the Middle East and violence has spilled over into Lebanon, a country that was itself torn apart by a bloody civil war from 1975-90.
Hezbollah's direct military intervention in the Syrian conflict in support of the regime of Bashar al-Assad has further fuelled ethnic tensions in Lebanon.
Assad and his ruling elite belong to the Alawite Muslims, a Shia offshoot, while rebels are predominantly Sunni.
Chatah death comes days before a UN-backed trial over the 2005 murder of Rafik Hariri is due to begin.
Hariri's Future Movement has long claimed that the Syrian Military Intelligence was behind the murder. Syria denies any involvement.