Muammar Gaddafi's son Saif al-Islam has been captured in Libya's southern desert, scared and with only a handful of supporters, by fighters who vow to hold him in the mountain town of Zintan until there is a government to hand him over to.

Crowds across the country fired guns and hooted car horns to celebrate the seizure of the British-educated 39-year-old, who was once seen as a future ruler of the oil-producing desert state.

The capture was the final act of the Libyan drama, said a spokesman for the country's interim government, nine months after the start of the uprising that ended Muammar Gaddafi's brutal one-man rule.

Fighters from Zintan said they stopped Saif al-Islam overnight as he drove through the desert in a small convoy and detained him without a fight. They flew him to their western mountain home on Saturday, accompanied on the plane by Reuters reporters.

Hundreds of people crowded round the plane when it landed, trapping him inside for more than an hour and raising fears he might suffer a similar fate to his father, who was beaten and shot after his capture a month ago on Sunday.

The Zintan fighters stopped people forcing their way on to the aircraft, bundled Saif al-Islam through the jostling crowd into a car and drove him away, to a secret location to protect his safety, they said.

Saif al-Islam's fate will be a key test for Libya's incoming government, due to be named on Monday according to sources, as it sets out to stamp its authority over a vast country, currently dominated by the armed militias who led the uprising.

Western leaders urged Libya to work with the International Criminal Court (ICC), which has also issued an arrest warrant for Saif al-Islam, on charges of crimes against humanity during a crackdown on protesters.

Amnesty International and Human Rights Watch both called on Libya to hand over Saif al-Islam to the global court, based in The Hague, and guarantee his safety.

But Libya's interim justice minister Mohammed al-Alagy told Reuters Saif al-Islam would be tried inside Libya for serious crimes that carry the death penalty.

Prime Minister-designate Abdurrahim El-Keib said Libya would make sure Gaddafi's son faced a fair trial and called his capture the crowning of the uprising.

We assure Libyans and the world that Saif al-Islam will receive a fair trial ... under fair legal processes which our own people had been deprived of for the last 40 years, Keib told a press conference in Zintan.

Saif al-Islam, who had vowed to die fighting, was taken without a struggle, possibly as he tried to flee to Niger, officials said.

At the beginning he was very scared. He thought we would kill him, Ahmed Ammar, one of his captors, told Reuters.

Saif al-Islam told a Reuters reporter on his plane his bandaged hand had been wounded in a NATO air strike a month ago. Asked if he was feeling all right, Gaddafi said simply: Yes.

The Zintan fighters, who make up one of Libya's most powerful militia factions that hold effective power in a country still without a government, said they planned to keep him in Zintan until they could hand him over to authorities.

Incoming premier Keib heaped praise on the militia and said Gaddafi's son remained in the hands of the revolutionaries in Zintan, acknowledging the authority the militia continued to hold over its territory.

Zintan could now use Saif al-Islam as a bargaining chip in the contest between rival groups for power in the new Libya. Fighters from Zintan made the decisive push on to Tripoli which ended Muammar Gaddafi's rule, and they want to make sure their contribution is recognised.

Libyans believe Saif al-Islam knows the location of billions of dollars of public money amassed by the Gaddafi family. His captors said they found only a few thousand dollars and a cache of rifles in seized vehicles.

Ammar told Reuters that his unit of 15 men in three vehicles, acting on a tip-off about a possible high-profile fugitive, had intercepted two cars carrying Gaddafi and four others in the desert about 70 km from the small oil town of Obari at about 1:30 a.m. (23:30 British time on Friday).

SERVANT OF PEACE

After the fighters fired into the air and forced the cars to stop, they asked the identity of the passengers. Saif al-Islam replied said he was Abdelsalam - a name that means servant of peace said the fighters who recognised and seized him.

The fighters said they put him at ease and he accepted he would be taken to Zintan, a town south of Tripoli that was a stronghold of anti-Gaddafi rebels.

Saif al-Islam appeared relatively at ease and was not handcuffed as he sat on a bench at the rear of the plane.

Wearing traditional robes with a scarf pulled over his face, Saif al-Islam had a heavy black beard and wore his rimless spectacles.

His thumb, index and another finger were heavily bandaged from the wounds sustained in the NATO strike.

Muammar Gaddafi's beating, abuse and ultimate death in the custody of former rebel fighters was an embarrassment to the previous transitional government. Officials in Tripoli said they were determined to handle his son's case with more order.

The capture presents a challenge to the NTC. If they want to try Saif then what can they do to make Zintan hand him over? said Henry Smith, an analyst with the Control Risks group, referring to the National Transitional Council which won international recognition as Libya's new interim government.

Memories are still fresh of the days Gaddafi's father's corpse spent rotting and on public view in the city of Misrata, another rebel stronghold, as its militia leaders trumpeted their capture of the fallen leader as part of their campaign to extract power and patronage from the new interim authority.

A fighter from an anti-Gaddafi unit, the Khaled bin al-Waleed Brigade, which said it seized Saif al-Islam in the wilderness near the oil town of Obari, told Free Libya television: We got a tip he had been staying there for the last month.

They couldn't get away because we had a good plan, Wisam Dughaly added, saying Saif al-Islam had been using a 4x4 vehicle: He was not hurt and will be taken safely for trial so Libyans will be able to prosecute him and get back their money.

We will take him to Zintan for safekeeping to keep him alive until a government is formed and then we will hand him over as soon as possible, Dughaly said.

He added that Saif al-Islam, once seen as a reformer who engineered his father's rapprochement with the West, appeared to have been hiding out in the desert since fleeing the tribal bastion of Bani Walid, near Tripoli, in October.

I'm really surprised that Saif al-Islam has not met the same fate as his father and his brother, Fawaz Gerges, professor of international relations at the London School of Economics, told BBC TV.

The best thing that the new leadership can do is to hand Saif al-Islam to the International Criminal Court because I don't believe it really has the resources and the means to try Saif al-Islam and give him a fair trial.

ALMOST ZERO

Asked of the chances of that, he said Almost zero. He said he expected him to get the death penalty and be executed in Libya. This is unfortunate for the new Libya, he said.

Justice Minister Alagy said he was in touch with the ICC over how to deal with Gaddafi.

We Libyans do not oppose the presence of international monitors to monitor the trial procedures that will take place for the symbols of the former regime, he told Al Jazeera.

Other Libyan officials have said a trial in Libya should first address killings, repression and theft of public funds over the four decades of the elder Gaddafi's personal rule.

There was no word of the other official wanted by the ICC, former intelligence chief Abdullah al-Senussi.

The European Union's foreign policy chief Catherine Ashton hailed the arrest of Saif al-Islam as a significant development and told Libya's new rulers to ensure full cooperation with the ICC. [nL5E7MJ0JD]

British Prime Minister David Cameron, who with France pushed for foreign military intervention in Libya, joined calls for a fair trial and offered Libya help in ensuring justice.

The Libyan government has told us again today that he will receive a trial in line with international standards, and it is important that this happens, he said in a statement.

(Additional reporting by Ismail Zeitouny and Mahmoud al-Farjani in Zintan, Ali Shuaib, Alastair Macdonald, Omar Younis, Hisham El-Dani in Tripoli, Francois Murphy in Benghazi, Erika Solomon in Beirut, Christian Lowe in Algiers, Peter Apps and Michael Holden in London, Nicholas Vinocur in Paris and Gilbert Kreijger in Amsterdam; Writing by Alastair Macdonald, Peter Millership and Andrew Heavens)