23 August 2011 Last updated at
Profile: Saif al-Islam Gaddafi
Saif al-Islam, whose name means Sword of Islam, has always denied that he was seeking to inherit power from his father
Despite holding no official position in the Libyan government, Saif al-Islam Gaddafi has long been seen as the second most influential figure in the country after his father.
But he is now wanted by the International Criminal Court accused of crimes against humanity for his alleged role in the suppression of opposition protests in February.
Until the rebellion - and the government's response - this stylish English-speaker, 39, was considered the reformist face of the Libyan government.
The second of Col Muammar Gaddafi's nine children has also made several speeches vowing to fight to the end against the rebels and accusing them of being "drunkards and thugs", as well as "terrorists".
"We are in our land and in our country. We will resist for six months, one year, two years... and we will win," he said in a speech broadcast on state TV on Sunday, just hours before rebel forces entered Tripoli.
The rebels had claimed to have captured him during their advance and the ICC said it was looking forward to putting him on trial.
But he then turned up outside a Tripoli hotel where international journalists are based, greeting crowds of cheering supporters.
And he still sounded confident of victory, saying the rebels had been ambushed by forces loyal to his father.
"We broke the backbone of the rebels," he said. "It was a trap. We gave them a hard time. So we are winning."
It is not clear whether he had been detained and managed to escape or whether the claims were simply not true.
He had previously argued that Libya needed democracy and played a key role in Libya's rapprochement with the West between 2000 and the 2011 uprising.
As head of the Gaddafi family's charity and allegedly the multi-billion dollar sovereign wealth fund, the Libyan Investment Authority (LIA) - although he denies this - Saif al-Islam had access to huge amounts of money, which he used to smooth relations with the West.
Who is Saif al-Islam?
Stylish, English-speaking bachelor
Long seen as likely successor to his father - suggestions he played down
Previously seen as reformist face of his father's regime
Played key role in rapprochement with the West from 2000-2011
Accused of organising brutal crackdown on opposition protests in February
He was involved in the negotiations which led his father to abandon his nuclear weapons programme and later helped mediate the release of six Bulgarian medics accused of infecting children with HIV in a Libyan hospital.
The trained engineer also negotiated compensation for the families of those killed in the 1988 Lockerbie bombing, the 1986 Berlin nightclub attack and the 1989 downing of UTA flight 772.
Again, he is said to have been involved in talks about the controversial 2009 decision to free from prison the man convicted of the Lockerbie bombing, Abdelbaset al-Megrahi.
After these agreements, international sanctions were lifted and with Saif al-Islam prominent both politically and economically, Libya seemed set to embark on an era of remarkable change.
Its oil sector was starting to open up and Libya had agreed to tackle the growing flow of sub-Saharan African migrants through the country to Europe.
The bachelor owns a house in London and has had links to British political figures as well as the royal family. He has met the Duke of York many times and is said to have visited both Buckingham Palace and Windsor Castle.
He is known to keep two tigers as pets and also enjoys hunting with falcons in the deserts - a past-time traditionally enjoyed by Arab royals - and is a keen amateur painter.
Saif al-Islam, whose name means Sword of Islam, always denied that he was seeking to inherit power from his father, saying the reins of power were "not a farm to inherit".
He had announced his retirement from politics in a 2008 speech.
"I intervened extensively in everything: Our foreign policy, in a lot of problems, in development, in housing. Because there were no institutions or an administrative system that were able to do so," he said.
"But now the situation has changed and if I continue there will be a problem."
He also called for political reform - a theme he addressed in the doctorate he obtained the same year from the London School of Economics (LSE).
When Saif al-Islam's role in the crackdown against protesters was reported, LSE director Howard Davies resigned from his post after facing criticism for accepting donations from the charitable foundation led by the son of the Libyan leader.
The university is also investigating the authenticity of Saif al-Islam's PhD thesis, amid reports it was plagiarized.
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