Several loud explosions followed by heavy gunfire were heard over Tripoli in the early hours of Wednesday, a Reuters reporter said.
The cause of the firing was not immediately clear, but the Libyan capital has been on edge amid speculation that a U.N.-backed unity government was about to travel to Tripoli from Tunisia.
The self-declared government in Tripoli and some armed factions in the city oppose the unity government and have warned it not to move.
The Tripoli government was brought to power after armed brigades backing it won a battle for the capital in 2014. A rival government moved to eastern Libya.