The clashes follow weeks of heightened tension in Lebanon
Four people were killed and more than 150 injured, police said.
It comes two days after three people died in clashes amid a general strike called by the militant Hezbollah group.
In Paris, foreign donors pledged $7.6bn (£3.5bn) to help Lebanon recover from last year's conflict between Hezbollah and Israel and a huge public debt.
The biggest pledges came from Saudi Arabia, the US, France and the EU.
Lebanese Prime Minister Fouad Siniora, who attended the conference, said he was "really pleased with the level of financial support".
However, the BBC's Caroline Wyatt in Paris says the Lebanese government had to promise to implement potentially unpopular economic reforms, which could create further difficulties with the Hezbollah-led opposition.
Campus brawl
The latest violence started as a row between Sunni supporters of the government and Shia opponents at Beirut's Arab University but it flared rapidly from a student fist fight to violent clashes between local supporters of the two sides.
Troops were called in to try to separate the two sides
Television pictures showed youths moving through the streets, brandishing makeshift weapons and vandalising cars.
As the row escalated supporters of the Shia Hezbollah movement called in help, and residents from the local Sunni neighbourhood also joined in.
Armoured vehicles full of soldiers moved in, firing shots in to the air, trying to keep the two groups apart.
The clashes erupted in a volatile area where the mainly Sunni population overlaps with Shia neighbourhoods.
Gunfire continued to echo in the area after nightfall but police later said order had been restored.
The clashes reinforced fears raised by the general strike on Tuesday that a major flare-up of civil strife could break out if urgent action is not taken to defuse the explosive political situation, reports the BBC's Jim Muir in Beirut.
Hezbollah, which is backed by Syria and Iran, has led mass demonstrations and strike action since the beginning of December to try to force Mr Siniora's pro-Western government to resign.
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Fights between students in Beirut Arab University esacalated into clashes across the city on January 25, between groups supporting and opposing the Lebanese government. Four people were killed while and over a hundred and fifty were injured in the fighting, as youths armed with makeshift weapons clashed, threw rocks and damaged vehicles. The Lebanese army deployed armored vehicles and fired shots in the air to disperse the fighters. An overnight curfew was imposed to stop the fighting.

Scuffles broke out between Sunni and Shiite students, who were respectively support and oppose of the current government, in the university's cafeteria and soon attracted local supporters of both groups. 

Coming on the heels of mass demonstrations and a general strike called by Hezbollah calling on the Fouad Siniora-led government to step down, the clashes has raised fears of renewed civil conflict along sectarian lines in Lebanon.