The EFCC accuses him of involvement in a 6bn naira ($40m) fraud from his time as governor of Sokoto State.
At the time of his arrest he was in a meeting trying to form an opposition party for an election due in 2011.
His allies have accused the authorities of trying to scuttle their plans to unite against the ruling party.
Also at the meeting was another former presidential candidate Muhammadu Buhari, and ex-Vice-President Atiku Abubakar.
Opposition groups have accused the EFCC (Economic and Financial Crimes Commission) of being a tool for the government to silence its critics.
But the EFCC argues that it has a good record - securing dozens of convictions and recovering billions of dollars.
