Nigeria's leading opposition candidate has warned of mass action if upcoming presidential elections are rigged by the government.

Muhammadu Buhari of the All Nigeria People's Party (A-N-P-P) told reporters Wednesday that any repeat of what he called "the fraud" of last Saturdays' parliamentary vote would result in unforeseen consequences.

He also called on Nigeria's electoral commission to re-run certain parliamentary races but stopped short of boycotting Saturday's presidential poll.

With more than two-thirds of the vote declared, the ruling People's Democratic Party of President Olusegun Obasanjo is set to retain an absolute majority in both the house and senate.

Although the ruling party has a commanding lead, some top ruling party officials lost their bid for re-election to opposition candidates.

On Tuesday, the chairman of the All Nigeria People's Party (A-N-P-P) called last Saturday's parliamentary balloting "flawed." He said the opposition had rejected the results. The chairman made the comments after a meeting of 28 opposition political parties.

Although the vote was deemed free and fair by international monitors, at least 12 people were killed in election-related violence. There have also been charges of voter intimidation and fraud.

On Saturday, President Obasanjo faces 19 candidates in the presidential election. His most serious challenger is Mr. Buhari.

Nigeria's elections are the first organized by a civilian government in 20 years.