Libya's Interior Ministry has said that the capital's police chief was killed as fighting continued for control of Tripoli airport.
The ministry said that Colonel Mohammed Sweissi was found dead on Tuesday.
Its statement said Colonel Sweissi came under attack by masked armed men after leaving a meeting in the Tajoura neighborhood.
State news agency Lana confirmed the assassination and said Colonel Sweissi died shortly after arrival at a nearby clinic.
The agency added that the two bodyguards who had been abducted when Sweissi was killed had been freed and were heading back to their Tripoli headquarters. It gave no further details.
The interior ministry said an investigation was under way into the killing, which took place as armed groups from western city of Misrata and their Islamist allies battled a armed group from the mountain town of Zintan over the airport.
The two militias played key roles in the overthrow of Muammar Gaddafi. There is also fighting in the eastern city of Benghazi, between Islamist armed groups and remnants of the army, which Sweissi backed.
Popular vote for president
Meanwhile, Libya's new parliament agreed on Tuesday that the next president would be elected by popular vote.
We have made the decision on electing the president directly, but we have not set a date for the vote until we
discuss the current crisis and see how stable the situation is
The House of Representatives voted overwhelmingly for the new president to be directly elected by the Libyan people as it seeks to put the country back on track towards democracy, three years after Gaddafi was overthrown.
No date has been set for the election.
"We have made the decision on electing the president directly, but we have not set a date for the vote until we discuss the current crisis and see how stable the situation is", lawmaker Fatthallah Saiti told Reuters news agency.
Libya's fragile government has been caught in political infighting that paralysed the last parliament and empowered rival brigades of heavily armed ex-rebels.
Those factions have clashed in the past, but last month their rivalries erupted into heavy street battles that are part of a wider struggle over the spoils of post-Gaddafi Libya.
Most Western diplomats have pulled out of Libya and closed their embassies, fearing the North African oil-producing state is edging closer to another war.
A United Nations delegation has been holding talks with the two factions in an attempt to broker a ceasefire.
The UN mission in Libya, known as UNSMIL, called on both sides to respond positively to efforts to end bloodshed.
"UNSMIL strongly condemns the continuing battles in Tripoli despite the repeated official and international calls for an
immediate ceasefire and to refrain from the use of force to resolve political differences," the mission said in a statement.