Senussi, among the most feared members of Gaddafi's regime before rebels toppled it last year, was captured in the West African state in March, triggering a tug of war between Libya, France and the International Criminal Court for his extradition.
"The office of the prosecutor general has received Senussi and he will undergo a number of medical tests. Soon he will also undergo interrogation for the cases he has been charged with," prosecutor general spokesman Taha Ba'ra told Reuters.
A heavily bearded Senussi, recognisable from his characteristic tightly curled hair, was shown in a press photograph getting out of a helicopter in Libya. Smiling slightly, a soldier stood behind him steadying him on the steps.
His extradition came after a high-level Libyan delegation, including the justice minister and army chief of staff, visited Mauritania on Tuesday and, a Mauritanian government source told Reuters, gave guarantees concerning Senussi. It was not clear if there was a court decision sanctioning the extradition and the source declined to specify the nature of the guarantees.
Senussi was arrested six months ago after arriving with a falsified Malian passport on a flight into the Mauritanian capital Nouakchott from Morocco. Mauritania's original plan was to put him on trial for illegal entry - a move that threatened to delay efforts to have him face international justice.
A spokesman for the ICC, which has wanted to try Senussi on charges of crimes against humanity including murder and persecution, said it had received no information about a handover to Libyan authorities in Tripoli.
In its warrant for Senussi's arrest, the Hague-based ICC said he had used his position of command to have attacks carried out against opponents of Gaddafi, who was hunted down and killed by rebels after his ouster in August last year.
France has wanted to try Senussi in connection with a 1989 airliner bombing over Niger in which 54 of its nationals died.
Senussi has also been linked to the 1988 bombing over Lockerbie, Scotland of an American PanAm jet that killed 270 people. Diplomatic sources have said the United States was keen to question him about that attack.
Gaddafi's son, Saif al-Islam, is to go on trial in Libya this month, a government source said in August, in what would be the most high-profile prosecution of a figure from the late dictator's 42 years in power.
Libya's new rulers, who aim to draw up a democratic constitution, are keen to try Gaddafi's family members and loyalists at home. But human rights activists worry that a weak central government and a relative lack of rule of law mean legal proceedings will not meet international standards.
Saif al-Islam is also wanted by the ICC for crimes against humanity during the uprising that brought down his father.
