Libyan militia groups opposed to the UN-backed government have launched a series of attacks on loyalist forces in the capital, Tripoli.
At least 28 people were killed and about 130 injured, officials said.
Explosions were heard across the city and witnesses said residential areas in the south had been shelled.
The UN-backed Government of National Accord (GNA) has struggled to establish itself in Tripoli since it arrived there in March 2016.
It wields little control beyond parts of the capital and relies on a complex network of armed groups with shifting allegiances.
Friday’s fighting began at dawn around a complex of luxury villas in the south of the city.
Militias opposed to the GNA said they had attacked loyalist forces.
The health ministry was not able to say if civilians were among the dead but reports suggested most casualties were fighters.
Until recently, the area had been the headquarters of militias supporting former Prime Minister Khalifa Ghweil. He was ousted when the GNA was set up and refused to recognise the new administration.
Forces loyal to the GNA seized the complex in March but dozens of armed groups still operate there.
The internationally recognised authorities condemned the fighting, saying "there was no room left for anarchy and chaos".
Also in southern Tripoli on Friday, an armed group loyal to the GNA reportedly seized al-Hadhba prison where former officials of ousted dictator Col Muammar Gaddafi are held.
Judicial sources quoted by AFP news agency said prison guards were forced to withdraw and two were killed.
The fate of the inmates is not clear. The prison is run by an Islamist group which has not yet declared whether or not it supports the GNA.
Libya has been in chaos since the overthrow of Gaddafi in October 2011. There are two rival parliaments and three governments.
The years of turmoil have allowed the Islamic State group to gain a foothold in the country.