The interim government led by prime minister Abdullah al-Thani, which resigned last week and has taken refuge in the east of the country, said armed groups were preventing government workers from entering their offices.
"Ministry and state offices in Tripoli have been occupied by armed militias who are preventing government workers from entering and are threatening their superiors," the government said in a statement released overnight.
It said the interim government was in contact with officials and "trying to ensure the continuity of services from afar."
The interim government announced last week it had tendered its resignation to the elected parliament, days after a rival Islamist administration was created.
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The parliament is also operating out of Libya's east for security reasons.
A rival body, the General National Congress, last week named pro-Islamist figure Omar al-Hassi to form a "salvation government".
Interim authorities have been steadily losing ground to the militias and the Fajr Libya (Libya Dawn) mainly Islamist alliance, which has seized Tripoli airport after weeks of fierce fighting with nationalist rivals.