Gunmen on Sunday surrounded Libya's foreign ministry demanding it be "cleansed of agents" and ambassadors of ousted dictator Moamer Kadhafi and warned they will spread their protest if their demands are not met.
The group prevented staff from entering the building in Tripoli, AFP correspondents said.
Around 30 vehicles, some mounted with anti-aircraft guns, and dozens of armed men surrounded the office, they reported.
An official, who declined to be named, admitted that the gunmen's demands were "legitimate" but he said it did not justify "paralysing the whole work of a ministry" and criticised the group's "extremely offensive" behaviour.
The gunmen said they wanted the ministry to be "cleansed of agents" of the former regime and stressed they will stand their ground until their demands are met.
They also warned they could spread their action to other ministries.
"Political exclusion is an obligation," one of the gunmen said, adding that former regime officials still hold key jobs at the foreign ministry.
The General National Congress, Libya's highest political authority, is studying proposals for a law to exclude former Kadhafi regime officials from top government and political posts.
The proposed law could affect several senior figures in the government, and has caused waves in the country's political class.
In March, demonstrators encircled the assembly, trapping members in 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.
Libya's government is struggling to assert its influence across the country, where former rebels who fought Kadhafi in the 2011 uprising still countrol much of the territory.