By Ghaith Shennib and Jessica Donati
TRIPOLI (Reuters) - Armed men in pick-up trucks mounted with anti-aircraft guns and rocket-launchers protested on the road outside Libya's justice ministry on Tuesday to press demands that former dictator Muammar Gaddafi's aides be barred from senior government posts.
Tensions between the government and armed militia have been intensifying since authorities began a campaign to dislodge the gunmen from strongholds in the capital Tripoli to tackle lawlessness threatening democratic transition.
Prime Minister Ali Zeidan's government and its armed forces are so weak that swathes of the oil-producing desert country are beyond their control. The militia, who played a pivotal role in the anti-Gaddafi revolt, have never left the capital.
Gunmen ringed the foreign ministry on Sunday and have targeted other state buildings, aiming to paralyze government until legislation is adopted banning those who served Gaddafi.
The legislation could blacklist several long-serving ministers, the congress chairman and Zeidan himself.
On Tuesday also, demonstrators marched to the General National Congress (GNC) building, or parliament, where they spray painted 'down with the regime' and 'no to Zeidan's government' in red on the front of the building.
The unrest has forced the GNC to postpone its next sitting, scheduled for Tuesday, to Sunday to give lawmakers time to consider the legislation that protesters want, a spokesman said.
"This is definitely an attempt to impose their agenda on the political process. It's not massively out of character - we have seen this before - but it is definitely a worrying trend," said a Western diplomat in Tripoli.
The armed men, with about 20 pick-up trucks, including one at the gates with Grad missiles, forced the minister and his staff to leave the building, one of the gunmen told Reuters.
Late on Tuesday, the road to the foreign ministry remained blocked by four-wheel drive cars and pick-up trucks loaded with anti-aircraft guns. Protesters have also unsuccessfully tried to storm the interior ministry.
About 100 people gathered in Tripoli's Martyrs Square on Tuesday to voice their support for the legislation, shouting, "Oh, martyrs, your blood will not go in vain", referring to those who died fighting to topple Gaddafi.
"If they don't pass the political isolation law, we will protest here and topple the government," said Faisal Alaqsa.
Demonstrators carried wooden coffins wrapped in flags and photographs of those who had died in the 2011 revolution.
Only a few dozen people turned up at different square for a rival demonstration in support of the government.
The U.N. Support Mission in Libya said it was monitoring the siege of state institutions and urged Libyans to resolve their differences through dialogue and abide by principles of democracy and rule of law that had driven the uprising.
(Editing by Louise Ireland)