Dozens of cars in a parking lot caught fire in the blast leaving several people trapped inside their vehicles and local TV broadcast images of at least one charred body. Body parts were scattered several meters from the scene.
The bomb targeted Lebanon's Internal Security Forces killing Captain Wissam Eid and his bodyguard. Eid participated in a military operation in May 2007 against the radical Fath al-Islam group.
Other Lebanese state-run media said at least six died and 18 were wounded.
The explosive device had been planted in an overpass in the Chevrolet neighborhood in Hazmieh, one of the major interchanges in northeast Beirut.
The explosion occurred a day after protests against the government's economic policies by transport and agriculture workers.
Lebanon has seen around 30 similar bomb attacks in the past three years, mostly aimed against Lebanese politicians and journalists.