Kadhafi, who once played professional football in Italy, would be held in accordance with "international standards regarding the treatment of prisoners," the government said.
The Tripoli Revolutionaries Brigade, a militia of former rebels, posted five photographs on Facebook of a disconsolate-looking Saadi wearing a blue prison uniform having his head and beard shaved.
The 40-year-old was pictured in the Libya Prison Authority photographs kneeling on the floor as he was being shaved by a man with an electic razor.
It released before and after photographs.
Kadhafi arrived on Wednesday night at Mitiga air base before being taken, manacled and blindfolded, to Tripoli's Al-Hadhba prison, Deputy Defence Minister Khaled al-Sherif said on Facebook.
A spokesman for the attorney general said he faces several charges, including "crimes to keep his father in power."
Seddik al-Sour told AFP the charges include involvement in the 2005 murder of a former coach of Tripoli football club Al-Ittihad.
He is also accused of "seizing goods by force and intimidation when he headed the Libyan Football Federation."
Saadi Kadhafi was best known as the head of Libya's football federation and a player who paid his way into Italy's top division.
The playboy footballer, born in May 1973, had been off the radar since fleeing in a convoy to Niger across Libya's southern desert in September 2011.
- Military career -
After hanging up his football boots, Saadi forged a military career, heading an elite unit.
Days after the revolt began in the eastern city of Benghazi, he appeared by his father's side in military uniform, a Kalashnikov assault rifle slung over his shoulder.
Unlike his brothers, however, no information emerged during the eight-month uprising of him taking part in combat.
Interpol had issued a "Red Notice" for Saadi, for "allegedly misappropriating properties through force and armed intimidation when he headed the Libyan Football Federation."
Libya had repeatedly called for Saadi's extradition from Niger, which had granted him asylum on "humanitarian" grounds saying it had insufficient guarantees he would have a fair trial.
Tripoli charged he was sowing sedition from exile.
Niger said it handed over Kadhafi because it no longer felt he would face the risk of extrajudicial killing, and because it wanted to improve ties with Tripoli.
Marou Amadou, the country's justice minister and government spokesman, said the political situation had "changed" in Libya.
He added that Niger had tried to find another country to take Kadhafi, but "we didn't find any candidate."
A group of NGOs in Niger condemned the extradition, warning that "the life of Saadi Kadhafi is under threat in Libya, which is a non-state with no security."
Human Rights Watch called on Niger to explain why they were convinced that Kadhafi would not be mistreated and would get a fair trial.
It said Libya has the responsibility to see that he is protected from torture or degrading treatment, is allowed visits from legal counsel, his family and medical personnel and promptly brought before a judge.
Three of Kadhafi's sons died in the 2011 uprising, including Mutassim who was killed by rebels in Sirte on October 20, the same day as his father.
Another son, Seif al-Arab, was killed in a NATO air raid in April 2011, and his brother Khamis died in combat in August at the height of the revolt.
But several key members of the Kadhafi clan survived. Among them were the dictator's erstwhile heir apparent, Seif al-Islam, wanted by the International Criminal Court but detained by a militia at Zintan in western Libya.
Two other brothers, former Libyan Olympic Committee chief Muhammad and Hannibal, who made headlines during scandal-packed European holidays, are believed to still be in Algeria, as is the fallen dictator's widow Safiya and daughter Aisha.
More than two and a half years after Kadhafi's downfall, Libya's transitional authorities have struggled to integrate former rebels into the police and army.
Security forces are regularly attacked, especially in the east, and the presence of rival ex-rebel groups, heavily armed with weapons looted from Kadhafi's arsenals, could yet push Libya into civil war.
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The Tripoli Revolutionaries Brigade, a militia of former rebels, posted five photographs on Facebook of a disconsolate-looking Saadi wearing a blue prison uniform having his head and beard shaved.
The 40-year-old was pictured in the Libya Prison Authority photographs kneeling on the floor as he was being shaved by a man with an electic razor.
It released before and after photographs.
Kadhafi arrived on Wednesday night at Mitiga air base before being taken, manacled and blindfolded, to Tripoli's Al-Hadhba prison, Deputy Defence Minister Khaled al-Sherif said on Facebook.
A spokesman for the attorney general said he faces several charges, including "crimes to keep his father in power."
Seddik al-Sour told AFP the charges include involvement in the 2005 murder of a former coach of Tripoli football club Al-Ittihad.
He is also accused of "seizing goods by force and intimidation when he headed the Libyan Football Federation."
Saadi Kadhafi was best known as the head of Libya's football federation and a player who paid his way into Italy's top division.
The playboy footballer, born in May 1973, had been off the radar since fleeing in a convoy to Niger across Libya's southern desert in September 2011.
- Military career -
After hanging up his football boots, Saadi forged a military career, heading an elite unit.
Days after the revolt began in the eastern city of Benghazi, he appeared by his father's side in military uniform, a Kalashnikov assault rifle slung over his shoulder.
Unlike his brothers, however, no information emerged during the eight-month uprising of him taking part in combat.
Interpol had issued a "Red Notice" for Saadi, for "allegedly misappropriating properties through force and armed intimidation when he headed the Libyan Football Federation."
Libya had repeatedly called for Saadi's extradition from Niger, which had granted him asylum on "humanitarian" grounds saying it had insufficient guarantees he would have a fair trial.
Tripoli charged he was sowing sedition from exile.
Niger said it handed over Kadhafi because it no longer felt he would face the risk of extrajudicial killing, and because it wanted to improve ties with Tripoli.
Marou Amadou, the country's justice minister and government spokesman, said the political situation had "changed" in Libya.
He added that Niger had tried to find another country to take Kadhafi, but "we didn't find any candidate."
A group of NGOs in Niger condemned the extradition, warning that "the life of Saadi Kadhafi is under threat in Libya, which is a non-state with no security."
Human Rights Watch called on Niger to explain why they were convinced that Kadhafi would not be mistreated and would get a fair trial.
It said Libya has the responsibility to see that he is protected from torture or degrading treatment, is allowed visits from legal counsel, his family and medical personnel and promptly brought before a judge.
Three of Kadhafi's sons died in the 2011 uprising, including Mutassim who was killed by rebels in Sirte on October 20, the same day as his father.
Another son, Seif al-Arab, was killed in a NATO air raid in April 2011, and his brother Khamis died in combat in August at the height of the revolt.
But several key members of the Kadhafi clan survived. Among them were the dictator's erstwhile heir apparent, Seif al-Islam, wanted by the International Criminal Court but detained by a militia at Zintan in western Libya.
Two other brothers, former Libyan Olympic Committee chief Muhammad and Hannibal, who made headlines during scandal-packed European holidays, are believed to still be in Algeria, as is the fallen dictator's widow Safiya and daughter Aisha.
More than two and a half years after Kadhafi's downfall, Libya's transitional authorities have struggled to integrate former rebels into the police and army.
Security forces are regularly attacked, especially in the east, and the presence of rival ex-rebel groups, heavily armed with weapons looted from Kadhafi's arsenals, could yet push Libya into civil war.
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