Mauritania arrested Muammar Gaddafi's ex-spy chief, Abdullah al-Senussi, after he arrived on an overnight flight, officials said on Saturday, triggering a three-way tussle for his extradition.

Senussi, who for decades before the late dictator's fall inspired fear and hatred in ordinary Libyans, is sought by the Hague-based International Criminal Court on charges of crimes against humanity during last year's conflict.

But Libya's new rulers insisted he would have a fair trial there, while France - confirming it played a role in his arrest - stressed his alleged role in the 1989 bombing of an airliner over Niger in which 54 French nationals died.

Today we confirm the news of the arrest of Abdullah al-Senussi, Libyan government spokesman Nasser al-Manee told a news conference in Tripoli.

He was arrested this morning in Nouakchott airport and there was a young man with him. We think it is his son, he said, confirming a Mauritanian state news agency report earlier that Senussi had been arrested with a false Malian passport arriving from Casablanca, Morocco.

France, which led Western backing for the popular uprising that toppled Gaddafi, said it had cooperated with Mauritanian authorities over the arrest and that it would send an arrest warrant to Mauritania in the next few hours.

A statement from President Nicolas Sarkozy's office noted Senussi had been sentenced in absentia for the 1989 bombing of a UTA airliner, in which a total 170 people were killed. Families of the victims immediately demanded he face justice in France.

For the time being, there is an ICC arrest warrant for him, and the court requests it to be implemented. This remains valid, unless the ICC judges decide otherwise, ICC spokesman Fadi El-Abdallah said.

However Mauritania is not a signatory to the Rome Statute governing the ICC and Libya's government spokesman Manee also confirmed it had sought the extradition of Senussi, the last key figure of Gaddafi's regime still at large.

The Libyan foreign ministry is in touch with Mauritania about the procedure. The Libyan government is ready to receive Abdullah al-Senussi...and give him a fair trial in Libya, he said.

Mohammed al-Harizy, spokesman for the National Transitional Council, also told a news conference that Libya would insist that Senussi be handed over to Libyan authorities.

We insist that Senussi is extradited to Libya, he said. There are demands from the ICC and France to get Senussi but the priority is to deliver Senussi to Libya.

Human Rights Watch said he should be handed over to the ICC.

Mauritania should support the Security Council's unanimous action by surrendering Senussi to the ICC, Richard Dicker, international justice director at HRW said in a statement. Senussi is a fugitive from justice, and Security Council members should also press Mauritania to hand him over to the court.

The Mauritanian government made no comment on the arrest beyond the report of its official news agency. The Casablanca flight normally arrives at Nouakchott's small, single-terminal airport just before midnight but workers there questioned by Reuters said they had not been aware of anything unusual.

A military source said Senussi had been taken to one of the residences in the grounds of Nouakchott's international conference centre which are used to house visiting officials, but it was not immediately possible to verify that information.

GADDAFI'S BLACK BOX

Senussi is suspected of a key role in the killing of more than 1,200 inmates at Tripoli's Abu Salim prison in 1996. It was the arrest of a lawyer for victims' relatives that sparked Libya's Arab Spring revolt in February last year.

The ICC has charged Senussi and Gaddafi's son Saif al-Islam as being indirect co-perpetrators of murder and persecution.

But Guillaume Denoix de Saint Marc, president of the families association for the UTA bombing, said they counted on France to ensure Senussi faced justice for the attack.

We never lost hope that those responsible for this attack, the most deadly that has hit France, would face justice, he said in a statement.

Senussi's arrest provoked equally fierce emotion on the streets of Tripoli.

Senussi is Gaddafi's black box, he has a lot of information, Tripoli resident Mustafa Jhyma said. He has blood on his hands he should be brought here and tried in Libya.

This is a big moment for Libyans. I wish that he had been arrested here, another resident Abdullah al-Mory said.

Saif al-Islam was captured disguised as a Bedouin in the Sahara in November is awaiting trial in Libya on rape and murder charges. Libya's National Transitional Council says he will get a fair hearing but his supporters want him sent to the Hague.

Mauritanian President Mohamed Ould Abdel Aziz, an army general who was toppled his predecessor in a 2008 coup, won election in a 2009 vote decried by rivals as rigged.

Yet France has hailed him as a key partner and he went on to play a leading role in the awkward African diplomacy over Libya that finally led to the continent recognising the National Transitional Council as its new leaders.

(Additional reporting by Roberta Cowan in Amsterdam, Marie-Louise Gumuchian in Tripoli and Lin Noueihed in Tunis; Maria-Victoria Buffery in Paris; Writing by Mark John; Editing by Giles Elgood)