During the fighting, a rocket-propelled grenade partially burned a military headquarters building. Three people were wounded.
The fighting underscores the volatility of Libya's security situation as a new leadership struggles to rein in armed groups, disarm former rebels and enforce the rule of law.
The two militias that clashed overnight Sunday control neighborhoods just a few streets apart.
The head of Libya's Supreme Security Committee, Mohammed Suweysi, says one of the militias attacked the other, accusing its members of detaining a man from their neighborhood and beating him to death.
He says the military has secured the area and made arrests.
(Copyright 2012 by The Associated Press. All Rights Reserved.)