Gunfighting between the two leading armed groups in Tripoli killed two people and forced the closure of the Libyan capital's only civilian airport, officials said on Tuesday.
The clashes between the influential 444th Brigade and the Rada Force, or Special Deterrence Force, erupted on Monday night and carried over into Tuesday, an interior ministry official said.
"Tensions arose" soon after it was announced "the Rada Force had arrested the head of the 444 Brigade, without explaining whether this was on judicial orders or for other reasons", the official said.
So far, two people had been killed and more than 30 wounded in the violence, a hospital source told AFP, as the fighting showed no signs of abating.
The United Nations Support Mission in Libya said in a statement it was "following with concern" the security deterioration in the Libyan capital and its impact on civilians.
"Violence is not an acceptable means to resolve disagreements," UNSMIL said.
"All parties must preserve the security gains achieved in recent years and address differences through dialogue," it added.
Images shared on social media late Monday showed armoured vehicles and armed pickups in the east and south of Tripoli after the arrest of 444 Brigade commander Mahmoud Hamza at Mitiga airport, in an area under Rada's control.
- Flights diverted -
Plumes of smoke were seen in Tripoli and gunfire was heard in the densely populated suburb of Ain Zara before it spread to areas near the airport and Tripoli University, which announced the suspension of classes.
The fighting was still underway on Tuesday and had forced "the closure of roads around Mitiga airport", according to the official.
Air traffic was stopped, flights were diverted to Misrata about 180 kilometres (110 miles) to the east, and planes that had been parked on the tarmac were moved away.
The health ministry called for blood donations and the establishment of a safe corridors to evacuate families trapped in the fighting.
The 444th Brigade is affiliated with Libya's defence ministry and is reputed to be the North African country's most disciplined.
It controls the southern suburbs of Tripoli as well as the cities of Tarhuna and Bani Walid, securing roads linking the capital to the south of the country.
The Rada Force, commanded by Abdel Raouf Kara, is a powerful ultra-conservative militia that acts as Tripoli's police force, arresting both suspected jihadists and common criminals.
It positions itself as independent of the interior and defence ministries, and it controls central and eastern Tripoli and Mitiga air base, the civilian airport and a prison.