Dozens of gunmen have occupied Libya's parliament to register their anger over the formation of the new government.
The militiamen are demanding some of the ministers be removed because they have links to the late Muammar Gaddafi's regime.
At least a dozen trucks mounted with anti-aircraft guns are lining the main road to the parliament.
Libya held a peaceful election in July and finally agreed the composition of a government on Wednesday.
Prime Minister Ali Zidan gained the support of the National Congress for his choice of ministers.
His list included liberal figures and Islamists in an attempt to forge a coalition acceptable to all parties.
But negotiations were disrupted by protests earlier this week.
And late on Wednesday gunmen broke through security and occupied the Congress building.
The BBC's Rana Jawad, in Tripoli, says some of the gunmen are dressed in scruffy army fatigues and others in civilian clothes.
She says some are from the western city of Misrata and others are from Tripoli, and few are willing to talk to the media.
"Some of them have had long ties with Gaddafi, we don't want them," a militiaman dressed in civilian clothes told the BBC.
Presidential guards are stationed in the Congress complex and have been ordered not to fight with the men.
The militiamen are believed to be in talks with politicians to resolve the stand-off.
Despite largely peaceful elections in July, Libya's transition continues to be affected by instability.
Reining in the different militia and trying to integrate them into a single national army will be one of the biggest challenges for any new government, analysts say.
The new government has representatives from the two most prominent blocs in Congress - the Alliance of National Forces led by liberal former Prime Minister Mahmoud Jibril, and the Muslim Brotherhood's Justice and Construction Party.
Mr Zidan said he had tried to strike a balance between Libya's different regions in making the appointments.
According to his list, the defence and interior ministries would be headed by ministers from the eastern city of Benghazi, considered to be the cradle of last year's revolution that ended Gaddafi's rule.
Two women are also among the ministers proposed by Mr Zidan.