Men in pick-up trucks bristling with anti-aircraft guns have blocked off Libya's foreign ministry, demanding a jobs ban on Gaddafi-era officials.
According to witnesses, dozens of armed men stopped workers entering the building and moved traffic away from the area.
Libyan officials have been unable to agree the terms of a law barring figures from the late Col Gaddafi's government from entering politics.
Armed groups have responded by storming the Congress on several occasions.
Prime Minister Ali Zeidan, at a news conference, urged Libyans to back their government in the face of "people who want to destabilise the country".