Armed clashes erupted in the centre of Tripoli on Tuesday, with unknown gunmen trading anti-aircraft and heavy machinegun fire, an AFP reporter said.
The fighting broke out at a building used as intelligence headquarters by the former regime of slain dictator Moamer Kadhafi, which was surrounded by former rebel fighters who toppled him last year.
There was no immediate clarity on the identities of the combattants.
Former rebels who remain organised in militia brigades are visible in many parts of the Libyan capital, and some have taken control of former government buildings, using them as headquarters.
Some witnesses near the scene told AFP that forces from the new government's interior ministry tried on Tuesday to retake the intelligence building, provoking a clash with the militia group that is currently occupying it.
While those reports could not be independently verified, the interior ministry declined to comment when contacted by AFP, saying only that it would comment on the clashes in the coming hours.
AFP sources said militiamen had blocked traffic from reaching the building, even as two ambulances tried to access the area of unrest, with witnesses reporting a number of injuries.
Many gunmen could also be seen circulating in the area and the situation remained chaotic, according to AFP reporters on the scene.