The attack was quickly claimed by the Abdullah Azzam Brigades, a jihadist group inspired by Al-Qaeda that previously claimed a double suicide bombing aimed at Iran's embassy in Beirut.
Jihadists have carried out a string of attacks in Lebanon targeting both Iran and the powerful Shiite movement Hezbollah, which provide vital support to Syrian President Bashar al-Assad's regime as it battles a Sunni-led rebellion.
The health ministry said six people were killed and 129 others wounded, with the army saying one of the cars was loaded with 90 kilos (198 pounds) of explosives and the other with 75 kilos.
Emergency teams carried wounded people away from a charred street strewn with rubble, as local residents armed with fire extinguishers helped firefighters put out blazes.
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The arms of a wounded man hung limply from the sides of a yellow stretcher as he was carried from the scene.
"I was driving my car with my wife to the university when we were hit by the force of the blast and I found pieces of human flesh on my face," Yousef al-Tawil, a professor at the Lebanese University, told AFP.
The Abdullah Azzam Brigades, an Al-Qaeda-inspired group, claimed the "double martyrdom operation" on Twitter and pledged to continue its attacks against Iran and "its party" -- a reference to Hezbollah.