Libyan PM Ali Zeidan has been seized from his hotel in the capital, Tripoli by a former rebel militia loosely allied to the government.
The group said it arrested Mr Zeidan following a prosecutor’s warrant, but the government has denied this.
He is being held at the interior ministry and being “treated well”, an official with the department said.
There is anger since Saturday’s US commando raid in Tripoli which seized senior al-Qaeda suspect Anas al-Liby.
Many saw the raid as a breach of Libyan sovereignty amid growing pressure on the government to explain if it was involved.
On Monday, Libya demanded an explanation from the US ambassador over the arrest of Mr Liby, who is wanted in the US over the 1998 bombings of US embassies in Kenya and Tanzania.
Libya’s government has been struggling to contain rival tribal militias and Islamist militants who control parts of the country, two years after the revolt which overthrew Muammar Gaddafi.
Correspondents say these militia often have the upper hand over police and army forces.
The Libyan government said on its website that Mr Zeidan had been taken from a hotel “to an unknown place for unknown reasons by a group thought to be from the Tripoli Revolutionaries Operations Room and the Committee for Fighting Crime”.
The government statement did not name the hotel, but a woman at the Corinthia Hotel – where the prime minister lives – confirmed the incident happened there when armed men entered the building.
She said no-one had been killed.
Al-Arabiya TV station broadcast images on Thursday morning which showed Mr Zeidan looking dishevelled and being escorted by what the station said were armed men.
Militias in Libya
- Numerous militias formed to topple Gaddafi still operate
- Many still control the towns or areas where they were formed
- Some believed to have links to al-Qaeda
- Government has been unable to disarm them, instead it works with some militias
- Libya Revolutionaries Operations Room, which says it seized the prime minister, has links to defence and interior ministries
- It condemned the US raid to seize al-Qaeda suspect Anas al-Liby
The BBC’s Rana Jawad, in Tripoli says there are a number of militia groups operating in Libya which are nominally attached to government ministries but often act independently.
One of them – Tripoli Revolutionaries Control Room – said it had arrested the prime minister, acting on the orders of the prosecutor general in accordance with Libya’s penal code.
However, state-run National Libyan TV quoted Justice Minister Salah al-Marghani as saying that the prosecutor general had issued no such order.
Libya’s cabinet has been summoned for an immediate meeting under the leadership of the deputy prime minister.
UK Foreign Secretary William Hague condemned the capture and called for Mr Zeidan’s immediate release.
“It is vital that the process of political transition in Libya is maintained. The government and people of Libya have our full support at this concerning time,” he said.
‘Act of sabotage’
Libyan politician Guma El Gamaty told the BBC that the raid against Mr Liby had sparked anger on the Libyan streets.
“Accusations have been pointed at the prime minister that there is some sort of a collusion – that the prime minister knew in advance that Abu Anas al-Liby was going to be kidnapped,” he said.
- Born 30 March 1964 in Tripoli, Libya. Also known as Nazih Abdul-Hamed al-Ruqai
- Believed to have joined al-Qaeda in 1990s
- Given political asylum in UK
- Rumoured to have returned to Libya during 2011 civil war
- Charged by New York prosecutors in 2000 with involvement in the 1998 Kenya and Tanzania US embassy bombings
- One of FBI’s “most wanted terrorists” with $5m bounty for his capture
A spokesman for the rebel group told Reuters news agency that they took Mr Zeidan following comments by US Secretary of State John Kerry that the Libyan government had been “aware of the operation”.
In an interview with the BBC on Monday, Mr Zeidan had said Libya was being used as a base to export weapons throughout the region, and called on the West to help stop militancy in Libya.
Last month Mr Zeidan visited the UK and appealed for British help to remove weapons from the country amid fears of increased arms smuggling to Syria.
In April he urged Libyans to back their government in the face of “people who want to destabilise the country”.
He also complained at that time of other attacks and “acts of sabotage” carried out by separate groups, against the interior ministry and national TV headquarters.