Gunmen demanding the sacking of former officials of the ousted Kadhafi regime surrounded the justice ministry on Tuesday, widening a campaign that began with a siege on 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 machineguns, anti-aircraft weapons and rocket launchers, saying 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 the national television station have also been attacked.
On Monday, angry police officers fired their guns in the air and 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".
For its part, the UN mission in the country on Tuesday urged all Libyans to engage in "constructive dialogue... in such a way as to promote the achievement of the Libyan revolution's goals".
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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 the General National Congress -- Libya's highest political authority -- adopts a bill calling for the expulsion of former regime employees from top government and political posts.
The bill could affect senior government figures and has roiled the political class.
Under pressure, Congress adjourned until Sunday to allow political blocs to seek consensus.
But some MPs said the real reason was security concerns over a planned demonstration on Tuesday outside the Congress.
Hundreds of supporters of the proposed Political Isolation Law gathered in Tripoli's Martyrs Square before marching on Congress, where they assembled peacefully in its courtyard under the watchful eyes of security, an AFP journalist said.
In March, demonstrators encircled the assembly, trapping members inside the building for hours as they called for the law's adoption.
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.
And security forces said about 100 common criminals escaped overnight from a prison in Sebha 800 kilometres (80 miles) south of Tripoli after sacking the facility.