University students loyal to Lebanon's government clashed with Hezbollah supporters, setting cars ablaze and battling with homemade clubs and stones.
Thursday's melee deepened worries that Lebanon cannot contain the political and sectarian rivalries threatening to push it toward civil war.
At least three people were killed and dozens were injured before army troops backed by tanks and firing barrages of warning shots into the air dispersed most rioters.
The military then declared Beirut's first curfew since 1996.
But the fallout reaches far beyond the casualty count.
The clashes, sparked by a cafeteria scuffle between pro-government Sunni Muslims and pro-Hezbollah Shiites, reinforced fears that Lebanon's sectarian divisions are erupting into violence as they did during the 1975-1990 civil war.
It was the third straight day of violence, sparked by a Hezbollah-led strike on Tuesday that came ahead of a crucial gathering of donor nations in Paris.
The conference on Thursday raised pledges of $7.6 billion to help Prime Minister Fuad Saniora's US-backed government rebuild after last summer's devastating Israel-Hezbollah war.
The money and show of international support could boost the embattled Saniora. The Iranian-backed Hezbollah has vowed to bring him down unless the opposition is given more power.