The bomb targeted Murr's motorcade in the busy Christian suburb of Antelias. Security officials said 12 people were also wounded in the explosion, audible for several km (miles).
LBC television station said one of Murr's aides was badly wounded in the attack.
Television pictures showed the smouldering wreckage of several cars almost totally destroyed in the blast. Blood stained the street and building facades were ripped off.
It was the latest in a series of bombings and assassinations in Lebanon since the Feb. 14 killing of former Prime Minister Rafik al-Hariri in a Beirut car bomb blast.
The latest explosion occurred as Prime Minister-designate Fouad Siniora was trying to form Lebanon's first government since Syrian troops withdrew from the country in April.
Although Murr is regarded as pro-Syrian, his father has forged a political alliance with anti-syrian Michel Aoun, a Christian former general who returned in May after 14 years of forced exile for leading a revolt against the Syrian military presence.
Aoun and his supporters won 21 seats in Lebanon's recent parliamentary election, the first since the Syrian pullout.