Nigeria's Vice-President Atiku Abubakar has ended months of speculation by announcing he will stand for the presidency in next year's elections.
Atiku Abubakar helped thwart a third term for President Obasanjo
However, Mr Abubakar did not tell the political rally in the capital, Abuja, what party he would be representing.
His candidacy is strongly opposed by President Olusegun Obasanjo and his governing People's Democratic Party.
Mr Abubakar fell out with Mr Obasanjo over the president's plan to amend the constitution and run for a third term.
The vice-president was suspended from the party earlier this year and has been accused of corruption.
Mr Abubakar denies allegations he diverted $125m into personal business interests.
Speaking to thousands of supporters at the rally, Mr Abubakar said an opposition candidate could win and that it was the people who decided elections, not the ruling party.
He said the corruption charges against him were politically motivated and designed to stop him running for the presidency.
The BBC's Alex Last in Nigeria says Mr Abubakar's speech gave the strongest hint yet that he may stand for a new opposition coalition, Action Congress.
But Mr Abubakar faces a difficult road to the presidency because elections in Nigeria are largely about money and power, and whoever is in government has plenty of both, our correspondent says.