Mauritania deports Libya spy chief Abdullah al-Senussi
Deposed Libyan leader Muammar Gaddafi's intelligence chief is being held in Tripoli after being deported from Mauritania.
Pictures on social media appeared to show Abdullah al-Senussi stepping down from a helicopter in the capital.
Libya has promised a fair trial for Mr Senussi, accused of crimes allegedly committed during Col Gaddafi's rule.
He fled Libya after last year's uprising. He is also wanted by France and the International Criminal Court.
"Abdullah al-Senussi will have a fair trial according to international standards for human rights, the rights from which Libyans were deprived," Prime Minister Abdurrahim el-Keib told reporters in Tripoli.
A spokesman for Libya's attorney general said Mr Senussi had undergone a routine medical check-up and was in good health. He added that the prosecutors would begin questioning him as soon as possible.
BBC Middle East editor Jeremy Bowen says a key question is what he might reveal about extraordinary rendition - the process under which jihadist enemies of Col Gaddafi's Libya were sent back to Libya by the US and Britain.
Some footage shot at a prison in Tripoli shows a heavily bearded man emerging from a helicopter to a crowd in military uniform chanting: "The blood of the martyrs will not go in vain."
It is in stark contrast to the clean-shaven, hardened face many Libyans grew accustomed to for decades.
Abdullah al-Senussi's extradition to Libya will set aside fears by many here that he will not face the justice so many are seeking.
The Libyan PM has made it clear that Mr Senussi will face trial in Libya, saying it will be fair and will meet international standards.
Both his extradition and any future trial are an opportunity for the new authorities to show that they are in control of security matters.
Mr Senussi was arrested on his arrival in Mauritania in March, sparking repeated requests to the west African nation from the Libyan government for his return.
"He was extradited to Libya on the basis of guarantees given by Libyan authorities," a Mauritanian government source told Reuters news agency, without giving details.
According to reports, Mr Senussi was delivered to an official Libyan delegation headed by the minister of justice.
The BBC's Jon Leyne in Cairo says Mr Senussi's extradition to Libya is a blow for the ICC.
Not only has the court been trying to win custody of Mr Senussi, he says, it is also arguing that Col Gaddafi's son Seif al-Islam should also be brought to justice at the court.Massacre
In March, Mr Senussi was arrested at Nouakchott airport in Mauritania after flying in from Morocco. He was disguised as a Tuareg chieftain and was carrying a fake passport.
He was later charged with illegally entering the country and using forged documents, and transferred to the civilian prison in Nouakchott. However, it is believed he has spent most of his time in Mauritania under house arrest at a private villa.
In June 2011, the ICC issued a warrant for Mr Senussi for crimes against humanity alleged to have been carried out in Benghazi, the main base of the Libyan opposition during the revolt last year.
France has already sentenced Mr Senussi to life imprisonment for his involvement in the bombing of a French airliner over Niger in 1989 in which 170 people were killed.
He has been accused of various human rights abuses including his alleged role in the 1996 massacre of more than 1,000 inmates at the Abu Salim prison in Tripoli.
He is alleged to have ordered guards standing on grated ceilings above the inmates to fire down on them, after riots broke out over demands for better food and conditions.
Mr Senussi is also believed to have information about Libyans kidnapped and assassinated abroad during Gaddafi's rule, and the financing of terrorist organisations, especially in Africa.
Investigators in the US and UK believe he may have further knowledge about the 1988 airliner bombing over Lockerbie in Scotland in which 270 people died.
Earlier this year, US House Minority Leader Nancy Pelosi, who led a delegation to the region, said Washington had a "particular interest" in seeing Mr Senussi arrested "because of his role with the Lockerbie bombing".