By Daniel Flynn

DAURA, Nigeria, April 22 (Reuters) - The main opposition candidate in Nigeria's presidential election said on Sunday he would not accept the result and called for President Olusegun Obasanjo to be impeached.

"We will not accept it. Clearly there was no election in more than half of the states," former military ruler Muhammadu Buhari told Reuters at his home in the northern state of Katsina.

Saturday's polls, like state governorship elections a week earlier, were marked by ballot stuffing, violence and intimidation, polling stations which never opened and a shortage of voting slips at those which did.

Independent and international monitors both said on Sunday the election was a failure.

Buhari, who lost elections in 2003 to Obasanjo, said he would meet other opposition leaders in the capital Abuja on Monday to agree a common front against the ruling People's Democratic Party (PDP).

He called for the election to be held again before May 29 when Obasanjo must stand down after serving two terms.

"There is a constitutional way out. The National Assembly, which has been recalled by the president of the Senate, should organise the impeachment of the president," he said.

Buhari accused sections of the military and the Independent National Electoral Commission (INEC) of colluding with the PDP to fraudulently ensure the victory of its candidate, Umaru Yar'Adua, Obasanjo's chosen successor.

"The PDP has no right to claim victory. If the PDP orders INEC to declare them the winners, then we will ask our supporters to stage peaceful protests at the convenient time," he said.

"It is likely to be a fatal blow to Nigerian democracy."

Buhari, the candidate for the All Nigeria People's Party (ANPP), said that if the situation was not resolved by the time Obasanjo is due to step down on May 29 then the country's chief justice should take over as interim president and organise new elections.

Nigeria's National Assembly is due to sit on Tuesday to discuss the elections.

Only a few hundred metres from Buhari's home, ANPP supporters clashed with soldiers on Saturday after they rioted to protest a shortage of presidential ballot papers.

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