Gunmen demanding the sacking of former officials of the ousted Kadhafi regime surrounded the justice ministry on Tuesday, stepping up an action started at the foreign ministry, an official said.
"Several armed men in vehicles equipped with anti-aircraft guns surrounded the ministry of justice," spokesman Walid Ben Rabha told AFP.
"They asked the minister and staff present to leave their offices and close the ministry."
An AFP photographer saw more than 20 pick-up trucks loaded with machine guns, anti-aircraft guns and rocket launchers and said they had blocked access to the building.
Dozens of gunmen making the same demand have kept the foreign ministry under siege since Sunday, paralysing its work.
The interior ministry and national television station have also been attacked.
On Monday, angry police officers firing their guns in the air stormed the interior ministry demanding higher wages.
Prime Minister Ali Zeidan has denounced the encircling of the foreign ministry and other such attacks.
On Sunday, he appealed to Libyans to support the government in resisting armed groups "who want to destabilise the country and terrorise foreigners and embassies," but said the government would "not come into confrontation with anyone."
The government is struggling to assert its influence across the country, where former rebels who fought to unseat Moamer Kadhafi in 2011 still control large amounts of territory.
The latest events illustrate a rise in violence in Tripoli, where a car bomb struck the French embassy last week, wounding two French guards and a girl living nearby.
Gunmen have said they will lift their siege when their demands are met by a vote in the General National Congress -- Libya's highest political authority -- on a bill calling for the expulsion of former regime employees from top government and political posts.
The bill could affect several senior government figures and has roiled the country's political class.
Under pressure, Congress adjourned until Sunday to allow its various political blocs to seek consensus on the bill.
That came as protesters gathered in central Tripoli's Martyrs Square and planned to hold a sit-in later Tuesday in front of the Congress.
In March, demonstrators encircled the assembly itself, trapping members inside the building for several hours as they called for the adoption of the law.
After the siege was lifted, gunmen targeted Congress chief Mohammed Megaryef's motorcade without causing any casualties.
In other developments on Tuesday, clashes raged in western Libya as tribes from the towns of Nalut and Tiji clashed with heavy weapons after an argument between two men, witnesses said.