The violence is the latest to rock the divided country since the toppling of long-time dictator Moammar Gadhafi in 2011. Since then, radical Islamists have clashed with western-backed authorities.
The death toll from intense clashes between government forces and militias in the Libyan capital has risen to 78, with as many as 1,000 injured, the government's health ministry reported Saturday.
The fighting broke out in Tripoli Friday between forces loyal to the UN-backed unity government and a rival militia loyal to an Islamist government that preceded the current one.
Khalifa Ghweil seized power in mid-2014 but was toppled in March 2016 by the unity government led by Fayez Serraj. Libya was thrown into chaos after a NATO-backed uprising brought down long-time dictator Moammar Gadhafi in 2011.
The latest clash started when the militia loyal to Ghweil, the self-appointed prime minister, attacked facilities manned by forces aligned with Serraj's Government of National Accord (GNA), according to Libyan media reports.
Fighting broke out in a residential neighborhood, around a complex of luxury villas that, until March, had served as the headquarters for militias loyal to Ghweil. There were reports of explosions and artillery fire in the Abu Slim, Al-Hadhba and Salaheddin districts in the south of the city.
Tripoli had been relatively calm since, but dozens of armed groups still operate there.
The United Nation's Libya envoy Martin Kobler appealed for a halt to the latest fighting.
"Voices of reason should prevail for the benefit of the country," he said. "Political aims must not be pursued through violence. Civilians must be protected."
Tanks and artillery
Witnesses said tanks were deployed during the fighting.
British ambassador Peter Millett tweeted that he could hear explosions and artillery in south Tripoli.
He condemned "action by these militias who threaten security" ahead of the Muslim holy month of Ramadan, which begins on Saturday in Libya.
Meanwhile, GNA forces seized a prison holding senior officials from the Gadhafi regime. Guards at the al-Hadhba prison were forced to withdraw after the attack.
The GNA's interior and justice ministries issued a joint statement saying that all the prisoners had been handed over to them and were "in good health."
Those imprisoned include more than 30 senior officials from the toppled regime, including Gadhafi's last prime minister, Baghdadi al-Mahmudi, and his former intelligence chief, Abdullah Senussi. Both men were sentenced to death in 2015.
The GNA blamed Ghweil and Salah Badi, both leaders of the Fajr Libya coalition of militias which took power in Tripoli in 2014, for the latest violence and vowed to "retaliate mercilessly."
They "have exceeded all limits... Nothing stops them, not faith, not law, not custom and not morals," it said.
"This is their gift to the people for the month of Ramadan," the statement said.
bik/rc (dpa, AFP, AP)