At least 31 people have been killed and 235 injured in clashes in the Libyan capital, Tripoli, officials say, after militiamen opened fire on protesters.
The demonstrators had marched to the headquarters of the Misrata militia to demand that it leave Tripoli.
Hours after the incident, armed men returned to storm the compound, where militiamen are still holed up.
The Libyan government has been struggling to contain numerous militias who control many parts of the country.
Prime Minister Ali Zeidan gave a televised address in which he said all militias had to leave Tripoli without exception.
However, it is unclear how the authorities plan to dislodge them, the BBC's Rana Jawad reports from Tripoli.
There have been increasing demands from civilians that the militias - which emerged during the 2011 revolution - disband or join the army, in line with an end-of-year deadline set by the interim government in Tripoli.
Mr Zeidan also blamed TV stations for inflaming the situation and advised them to exercise control over who was speaking on TV, since Libya did not have "a stable democratic situation" which would allow for freedom of expression.
"The demonstration was peaceful and had been permitted by the interior ministry, and then the protesters were fired on when they entered the Gharghur district," where the headquarters of the militia are located, Mr Zeidan told Reuters.
Saddat al-Badri, the leader of Tripoli's local council, who was at the initial protest, also said the demonstration was peaceful, but that the militiamen "fired their weapons as soon as we arrived, there was a 106mm used, and even an RPG".
"The protesters were not armed and they were all there chanting 'Libya'," he said, adding that the city risked descending into armed warfare.
The scene of the confrontation remained volatile and tense on Friday evening, our correspondent reports.
Military jets could be seen and heard flying low over the Airport Road area earlier in the day, and heavy gunfire was ringing out.
Witnesses said that armed men had stormed the militia HQ hours after the protesters were shot at, with some buildings set on fire.
Army vehicles have reportedly arrived to seal off roads and try to separate the combatants.
The militia was involved in clashes in the capital last week which left two people dead.
Last month, Mr Zeidan was briefly seized by a militia group in Tripoli.
Two years after the overthrow of Col Muammar Gaddafi, Libya still has no constitution and divisions between secular and Islamist forces have paralysed parliament.