Backers of the Tripoli government, which does not recognise the UN-backed GNA, stormed its offices on Friday night
Libya's UN-backed unity government suffered a blow in its Tripoli base late on Friday when a rival seized key offices in the capital and proclaimed the reinstatement of the former administration.
The Government of National Accord (GNA) was installed by the UN in April, intended to replace two rival administrations, one in Tripoli and one in the eastern Cyrenaica region.
Late on Friday, the head of the former Tripoli-based Government of National Salvation, Khalifa Ghweil, proclaimed his body's reinstatement after storming the offices of a key consultative body of the GNA.
Ghweil has never accepted the legitimacy of the UN-backed government, which began taking control of key parts of the country's administration in Tripoli in April.
He is subject to international sanctions, renewed by the European Union just last month.
In a statement issued on Friday night, Ghweil declared all members of the GNA "suspended from their duties".
The UN-backed government riposted with a statement threatening to arrest "those politicians who... attempt to create parallel institutions and destabilise the capital".
It condemned "efforts to sabotage the political agreement" brokered by the UN last December and denounced the seizure of the Council of State building by an "armed group".
The persistent chaos has hobbled efforts to battle a growing militant presence in Libya, which has been the launchpad of deadly attacks on holidaymakers in neighbouring Tunisia.
The western Tripolitania region had been the GNA's main stronghold.
The authorities in Cyrenaica also refuse to cede power, bolstered by the backing of the well-armed militia of controversial military strongman Field Marshal Khalifa Haftar.
The first, elected in 2012, is dominated by Islamists and appointed the Tripoli government.
The second, elected in 2014, is marred by a controversial court decision declaring its election illegal. It appointed the Cyrenaica-based administration.
The battle for power erupted into armed conflict last month, when the Cyrenaica administration's main backer, Haftar, seized all four of the main eastern oil export ports.
He exploited the absence of fighters loyal to the UN-backed government, who were battling IS in the city of Sirte to the west with air support from the United States.