Libya's prime minister hailed the capture of Muammar Gaddafi's son on Saturday as the crowning of the Libyan uprising and promised a fair trial for Saif al-Islam, who was found in the southern desert overnight.

In the first official announcement of Saif al-Islam's capture, Abdurrahim El-Keib said he hoped it would turn the page on the phase of revolution and will mark the beginning of the building of a state of freedom, law, justice and transparency.

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 mountain town of Zintan, where Saif al-Islam and several bodyguards had been taken.

Saif al-Islam, once favourite to succeed his late father, was arrested by fighters from Zintan, who make up one of Libya's most powerful militia factions. They said they would hold him until they could hand him over to the authorities.

The West urged Libya's new rulers to give Saif al-Islam a fair trial and work with the International Criminal Court to bring him to justice, fearing he might suffer the same fate as his father, who was beaten and shot dead after his capture.

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, said a spokesman for the European Union's foreign policy chief, Catherine Ashton.

Swedish Foreign Minister Carl Bildt pressed for his removal to the International Criminal Court in The Hague, which wants to try the 39-year-old on charges of crimes against humanity during the crackdown on protests.

The court's chief prosecutor, Luis Moreno-Ocampo, said he would visit Libya in a week to discuss the prosecution.

But many Libyans want Saif al-Islam tried at home, believing he knows the location of billions of dollars of public money amassed by the Gaddafi family.

Libya's interim justice minister said the country would try him first, for crimes that carry the death penalty.

We are ready to prosecute Saif al-Islam, Mohammed al-Alagy said. We have adopted enough legal and judicial procedures to ensure a fair trial for him.

Alagy, who does not expect to retain his post in a new government, said he would be tried on charges of instigating others to kill, misuse of public funds and recruiting mercenaries among other crimes.

Across Libya, Said al-Islam's capture was celebrated. Keib, the incoming prime minister, thanked Libyans for their struggle and historic heroism that ousted the regime and captured Saif al-Islam.

It is the crowning of the sacrifices of our people, he said.

(Reporting by Shaimaa Fayed, Erika Solomon and Marie-Louise Gumuchian; writing by Elizabeth Piper; editing by Janet Lawrence)