Story highlights The militants want senior government positions to be cleansed of Gadhafi loyalists

They have anti-aircraft guns mounted on their trucks, officials say

No violence was reported in the latest in a string of armed protests

The nation's Foreign Ministry remains under siege for a third straight day

Armed men in trucks with anti-aircraft guns mounted on them occupied the Libyan Justice Ministry in Tripoli on Tuesday, forcing ministry staff to leave, Justice Minister Salah al-Marghani said.

The militants consisted of 20 to 30 armed men in military fatigues, according to al-Marghani, who said he tried to talk to the men before fleeing.

This comes as the nation's Foreign Ministry remains under siege for a third straight day.

The armed protesters have said their main goal was to push the General National Congress to pass a proposed law that would ban Gadhafi-era officials from holding government posts.

The political isolation law proposal has been a matter of contention among lawmakers for several months because it could push current senior officials out of office for serving under the former regime.

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Watchdog groups have been calling on Libyan authorities to rein in armed groups that they say continue to threaten the country's future.

"Unlawful armed groups that show up with heavy weapons and block access to government institutions, demanding grievance, crosses the line of peaceful protest; it is intimidating and threatening and there should be accountability for these actions" Hanan Salah, the Libya researcher for Human Rights Watch, told CNN on Sunday.

In recent months, Libyans have resorted to armed protests in the capital. In some cases, protesters surrounded government offices, and sessions of the country's legislature have been interrupted by armed groups that stormed its meetings.

Last month, armed protesters besieged the General National Congress for several hours in an attempt to force its members to pass the political isolation law. Gunmen later opened fire on the vehicle of the parliament speaker, who escaped unharmed.

Eighteen months after the fall of the regime, Libya remains awash in weapons and militias that the government has been struggling to control to secure the country.