Abdullah al-Senussi, Libya's feared former intelligence chief, was cornered and captured at a remote desert homestead on Sunday, a day after Muammar Gaddafi's son was seized by Libyan fighters in the same region.

The arrest of the last survivor of the old regime who is wanted at The Hague for crimes against humanity crowned a momentous couple of days for a new government that is still in the process of formation, and also posed immediate tests of its authority -- both over powerful militias and with world powers.

In a sign of the strain that the prime minister-designate is under to reconcile the interests of rival militia groups that control the ground in Libya, officials said Abdurrahim El-Keib had asked for another couple of days to complete a cabinet that he had previously hoped to announce on Sunday.

A commander of former rebel forces nominally loyal to the National Transitional Council (NTC), General Ahmed al-Hamdouni, told Reuters that his men, acting on a tip, had found and surrounded Senussi at a house belonging to his sister near the town of Birak, about 500 km (300 miles) south of Tripoli and in the same region as Saif al-Islam was seized on Saturday.

NTC spokesman Abdul Hafez Ghoga later confirmed that Senussi, who is Saif al-Islam's uncle by marriage, had been captured. It was not immediately clear if the arrests were linked, though there has been speculation since the fall of Tripoli three months ago that the pair were hiding together.

Fighters who intercepted Saif al-Islam on a desert road in the early hours of Saturday said they believed one of his companions was also a nephew of Senussi, whose wife is a sister of Muammar Gaddafi's second wife Safiya.

Like Muammar Gaddafi, who was captured and killed on the coast a month ago on Sunday, Saif al-Islam and Senussi were indicted this year by the International Criminal Court for alleged plans to kill protesters after the Arab Spring revolt erupted in February.

But NTC officials have said they can convince the ICC to let them try both men in Libya.

Ghoga said NTC members meeting on Sunday had confirmed that preference, as did the current justice minister - although legal experts point out that international law demands Tripoli make a strong case for the right to try anyone who has already been indicted by the ICC.

MANY LIBYANS WANT HANGMAN'S NOOSE

Given the state of Libya's legal system after 42 years of dictatorship, as well as the depth of feelings after this year's civil war, the ICC seems unlikely to agree, many jurists think. Its chief prosecutor is expected in Libya this week.

While the ICC, backed by a U.N. resolution, can demand Libya hand over the prisoners, many Libyans are keen to see them tried for alleged crimes committed over decades, well beyond the scope of the ICC charges relating to this year only. And many also want them hanged, something barred at The Hague.

Among other old wounds, 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. And many of the dead were members of Islamist groups which are expected to be a major political force in a democratic Libya.

The case of Senussi, long the elder Gaddafi's right-hand man and enforcer, may also revive interest in international incidents long shrouded in mystery, from the days in the 1980s and 90s when Gaddafi's Libya waged undercover war on the West.

Senussi's name has been linked with the Lockerbie bombing of 1988. He was among six Libyans convicted in absentia in Paris of bringing down a French UTA airliner a year later.

Saif al-Islam Gaddafi spent Sunday at a secret location in the militia stronghold of Zintan while in Tripoli the Libyan rebel leaders who overthrew his father tried to resolve their differences and form a government that can try the new captive.

With rival local militia commanders from across the country trying to parlay their guns into cabinet seats, officials in the capital gave mixed signals on how long Keib, may need.

Ghoga said the NTC had given Keib another two days, right up to a deadline of Tuesday, to agree his cabinet -- a delay that indicated the extent of horse-trading going on.

And though the Zintan mountain fighters who intercepted the 39-year-old heir to the four-decade Gaddafi dynasty deep in the Sahara said they would hand him over once some central authority was clear, few expect Saif al-Islam in Tripoli soon.

Members of the NTC, the self-appointed legislative panel of notables formed after February's uprising, expect to vote on Keib's nominees, with keenest attention among the men who control the militias focussed on the Defence Ministry.

One official working for the NTC said that the group from Zintan, a town of just 50,000 in the Western Mountains outside Tripoli that was a stronghold of resistance to Gaddafi, might even secure that ministry thanks to holding Saif al-Islam.

Other groups include rival Islamist and secularist militias in the capital, those from Benghazi, Libya's second city and the original seat of revolt, and the fighters from the third city of Misrata, who took credit for capturing and killing the elder Gaddafi and haggled with the NTC over the fate of his rotting corpse for several days in October.

FINAL ACT

The final act of the Libyan drama, as a spokesman for the former rebels put it, began in the blackness of the Sahara night, when a small unit of fighters from the town of Zintan, acting on a tip-off, intercepted Saif al-Islam and four armed companions driving in a pair of 4x4 vehicles on a desert track.

It ended, after a 300-mile flight north on a cargo plane, with the London-educated younger Gaddafi, who had tried to pass himself off as Abdelsalam, a camel herder, being held in a safe house in Zintan and the townsfolk vowing to keep him healthy until he can face a judge in the capital.

His captors said he was very scared when they first recognised him, despite the heavy beard and enveloping Tuareg robes and turban he wore. But they reassured him and, by the time a Reuters correspondent spoke to him aboard the plane, he had been chatting amiably to his guards.

He looked tired. He had been lost in the desert for many days, said Abdul al-Salaam al-Wahissi, a Zintan fighter involved in the operation. I think he lost his guide.

Sitting on the tarmac at Zintan, under siege from a mob who seemed ready to inflict on him the indignities that met his father, revealed his fears, but also some bravado and not a little humour. When others in the besieged aircraft lit up cigarettes, he complained: We're going to choke to death.

In video posted on YouTube, he was later seen chatting in a room with others, apparently at ease in Zintan -- images that may surprise other Libyans who bear deep grudges against him.

There is no problem, he said at one point, after cursing the infidel Crusader pact of NATO whose air strike a month ago had killed 26 of his men and left him with a wounded hand.

How long Libya will hold on to him and Senussi, who officials said was being held overnight in the desert, was unclear. Despite official insistence, some analysts said Libya would face international pressure if it tried them itself.

Western leaders, who backed February's uprising against Gaddafi but looked on squeamishly as rebel fighters filmed themselves taking vengeance on the fallen strongman a month ago, urged Keib to seek foreign help to ensure a fair trial.

Keib, who taught engineering at U.S. universities before returning to Libya to join the rebellion, drove on Saturday the two hours from Tripoli to Zintan to pay homage to its fighters. He promised justice would be done - within Libya.

(Additional reporting by Marie-Louise Gumuchian, Hisham El-Dani and Francois Murphy in Tripoli and Oliver Holmes and Taha Zargoun in Zintan; Writing by Alastair Macdonald)