(Adds comments from Niger government spokesman)
By Ulf Laessing and Feras Bosalum
TRIPOLI, March 6 (Reuters) - Muammar Gaddafi's son Saadi, his special forces commander who fled abroad during Libya's 2011 revolution, was imprisoned in Tripoli on Thursday after Niger sent him back from house arrest there.
Saadi, who had a brief career as soccer player in Italy and lived a playboy life during his father's rule, is the first of Gaddafi's sons the central government has managed to arrest since the former ruler was overthrown.
Gaddafi's more prominent son Saif al-Islam, long viewed as his heir, has been held captive by fighters in western Libya who refuse to hand him over to a government they deem too weak to put him om trial.
Eager to close another chapter from Gaddafi's four-decade rule, Tripoli had long sought the extradition of Saadi, who had fled to Libya's southern neighbour by slipping over the sub-Saharan border after the uprising.
"The Libyan government received today Saadi Gaddafi and he arrived in Tripoli," Prime Minister Ali Zeidan's cabinet said in a statement that thanked Niger's government for its help.
The extradition is a success for Zeidan but also a test of whether his weak government is able to hold such a high-profile prisoner and organise a fair trial in the political chaos that has followed the uprising.
The government said Saadi, 40, would be treated according to international law.
Since escaping Libya in 2011, he had been held under house arrest in the Niger capital Niamey. Libyan authorities believe he was active from there in fomenting unrest in southern Libyan.
Within an hour of the news of his arrival, a militia on the Libyan state payroll published photographs of an uncomfortable looking Saadi in a blue prison jumpsuit, kneeling while a guard shaved his beard and head with an electric razor.
"The first pictures of the criminal," the Libyan Revolutionary Operations Room militia said on its website, showing pictures of Saadi before and after the shave.
State prosecutors are investigating Saadi for crimes in suppressing the eight-month uprising against his father, state news agency LANA said.
Tripoli also wants to try him for allegedly misappropriating property by force and for alleged armed intimidation when he headed the Libyan Football Federation.
ARREST MIGHT CALM SOUTH
Sources in Niger said Saadi had been taken to Niamey's airport late on Wednesday together with other exiles, where they boarded a Libyan plane accompanied by security agents.
Niger had refused Libyan requests to extradite Saadi for months, saying he might be killed, but a government spokesman said Niamey shifted position after a former top Libyan intelligence official, Abdallah Mansour, was well-treated following his repatriation last month.
"We extradited Saadi to foster an understanding between our two countries, which are friends and brothers," Justice Minister Amadou Marou told a news conference.
He said Saadi and other Libyans violated the terms of their exile, which specified they could not meddle in Libyan affairs.
Saadi's presence had proved an obstacle as Niger sought to step up security cooperation along their long border against weapons smugglers, militant Islamists and human traffickers.
Political analyst Khalid al-Tarjaman said Libya had convinced Niger that Saadi's presence was giving a boost to Gaddafi loyalists in Libya's volatile south whom Tripoli has accused of provoking clashes in the main city Sabha in January.
"The government in Niger realised that Saadi's presence was the main source for tensions in Libya's south, which is also affecting Niger's sovereignty," he said.
Saadi, who also had a business career before 2011 thanks to the quasi-monopoly his family enjoyed in many sectors of the economy, is not wanted by the International Criminal Court.
The ICC has indicted Saif al-Islam for crimes against humanity. He is being held by militia fighters in Zintan and tried there for various charges, although a local court keeps adjourning proceedings after brief sessions.
Zintan fighters have allies themselves in tribes that once formed Gaddafi's power base, which analysts say partly explains their benevolent attitude towards Saif al-Islam.
Tarjaman said Saadi's expulsion from Niger might help Libya persuade other countries such as Egypt or Tunisia to extradite Gaddafi relatives and former officials.
Several family members such as Gaddafi's daughter Aisha and her brother Hannibal had fled to Algeria during the uprising. They moved to Oman after Aisha had irritated Algerian authorities by discussing politics in public. (Reporting by Ulf Laessing and Ahmed Tolba; Additional reporting by Abdoulaye Massalaki in Niamey; Editing by Eric Walsh, Patrick Markey and Tom Heneghan)