Western leaders urged Libyan authorities on Saturday to work with the International Criminal Court in bringing the captured son of Muammar Gaddafi to justice, saying it was a vital step towards national reconciliation after the civil war.

Libya's Prime Minister-designate Abdurrahim El-Keib promised a fair trial for Saif al-Islam, who was captured in the southern desert overnight.

I want to assure our people and all nations of the world that Saif and those with him will be given a fair trial, with the guarantees of local and international law - those legal processes which our own people were deprived of, he told a news conference in the Western mountains town of Zintan, where Saif al-Islam was taken.

Fighters have vowed to hold him in Zintan until there is a government in place to hand him over to.

The European Union's foreign policy chief Catherine Ashton hailed the arrest of Saif al-Islam, once tipped to succeed his late father as Libyan leader, as a significant development and told Libya's new rulers to ensure full cooperation with the ICC.

It is important for future national reconciliation that those responsible for human rights violations committed both before and during the recent conflict are brought to justice, Ashton's spokesman Michael Mann said.

Sweden's Foreign Minister Carl Bildt called for the government to hand Saif al-Islam over to the ICC for trial.

The international court in June indicted Muammar Gaddafi, Saif al-Islam and intelligence chief Abdullah al-Senussi on charges of crimes against humanity over a crackdown on anti-government protests that eventually flared up into civil war.

Western leaders are eager to see Libya quickly form a competent government that can resume economic activity and prevent further violence. But after four decades of Gaddafi's authoritarian rule, Libya is riven with tribal animosities.

The ICC's chief prosecutor Luis Moreno-Ocampo is to visit Libya in a week to discuss Saif al-Islam's prosecution.

WESTERN ASSISTANCE

British Prime Minister David Cameron 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.

Britain will offer every assistance to the Libyan government and the International Criminal Court to bring him to face full accountability and justice for what he has done.

France, which together with Britain pushed for a military intervention in Libya last March, urged fighters who captured Saif al-Islam to hand him over to the authorities.

Saif al-Islam must answer for his acts and face trial, the French foreign ministry said.

Human rights activists said a trial by the ICC would send the right message to the international community that Libya is serious about protecting rights.

Fair prosecution at the ICC will afford Libyans a chance to see justice served in a trial that the international community stands behind, said Richard Dicker, international justice director at Human Rights Watch.

But some experts said the Libyan government was likely to opt for trial at home and the handling of the case would be a litmus test of the ability of Libya's new rulers to overcome divisions and give a foundation to credible state institutions.

If Saif is to go on trial in Libya it will be a major challenge for a country with barely any functioning governmental institutions to speak of. A prolonged trial may also bring divisions to the surface, said Alan Fraser, a Middle East analyst at London-based consultancy AKE.

Renewed violence in Libya or lengthy transition to democracy would tarnish the West's costly intervention in Libya. - a seven-month air and sea campaign by NATO that wound up at the end of October and was hailed by many Western leaders as a success for its role in helping overthrow Gaddafi.

A NATO spokeswoman said the alliance believed the authorities and the ICC would secure justice for Saif al-Islam.

We trust that the Libyan authorities and the International Criminal Court will ensure that justice runs its course so that the new Libya can be built on the rule of law and respect for human rights, said Oana Lungescu.

(Additional reporting by Peter Apps in London, Nicholas Vinocur in Paris and Helena Soderpalm in Stockholm; Editing by Janet Lawrence)