Lebanon's army has declared a curfew in Beirut after two people died in clashes between political groups at Beirut Arab University.
Pro and anti-government students hurled rocks and furniture at each other in running battles on the campus.
Parts of the building were set on fire.
The riot is the latest incident to ratchet up tension in Lebanon and will raise fears the country is once again slipping into civil war after a month of anti-government protests.
Hizbollah's al Manar television said one of the militant group's supporters was killed in fighting and security officials said at least 17 people were injured.
Soldiers fired shots in the air to disperse rioters.
The trouble in the Sunni Muslim neighbourhood of the capital spilled over to nearby streets, where people from outside the university joined in the fight.
Young men carrying sticks and wearing hard hats arrived in the area and started throwing stones at each other.
Some in the crowd smashed parked car windows before setting some on fire.
The battle was triggered by pro-government Sunni Muslims and supporters of the Shi'ite Hizbollah opposition movement in the university cafeteria, students said.
It follows big protests this week which saw parts of the country paralysed.
Shi'ite groups want more say in Lebanon's government, which it says is a front for US meddling.