The Lebanese army has imposed a dusk-to-dawn curfew in the capital, Beirut, after clashes between students from rival political factions.
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.
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.
Club-wielding students hurled rocks and other missiles at each other as fighting spread across the capital.
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.