ABUJA, Nigeria - Nigeria's election commission on Thursday rejected claims by the opposition and observers that widespread fraud marred recent legislative elections.

The dispute comes amid rising tensions in Africa's most populous nation ahead of another election Saturday for president. With more than three-quarters of the returns in, President Olusegun Obasanjo's ruling party has an outright majority in both the upper and lower chambers.

Questions also emerged over high turnout figures in the oil-rich Niger Delta, where ethnic militants boycotted the ballot last weekend and reporters saw few signs of voting for the 109-seat Senate and the 360-seat House of Representatives.

Several opposition leaders say the elections were rigged and have threatened to launch protests.

The president is running for re-election and has said he would use "all constitutional means" to maintain peace and order ahead of Saturday's vote.

Presidential challenger Emeka Ojukwu, a retired army officer who led Biafra rebels in Nigeria's devastating 1967-70 civil war, are among those calling for demonstrations.

"There is a limit to what people will believe," Ojukwu said of the elections.

Nigeria's main opposition leader, Muhammadu Buhari, also threatened "mass action" - a term in Nigeria generally used to refer to violent protests - if fraud taints the presidential vote.

Buhari, who like Obasanjo is a former military leader, launched a coup toppling President Shehu Shagari in 1983 after elections widely derided as flawed.

Abel Guobadia, head of the Independent National Electoral Commission, defended last weekend's vote and advised parties to file appeals with the federal courts.

But critics say the commission, which receives funds from the government, is biased.

The Roman Catholic Church's Justice, Peace and Development Center, Nigeria's largest election observer group, said the results do "not tally" with the reports of its 30,000 monitors across the country, adding that "somebody is fiddling with the figures."

The center's director, Ifeanyi Enwerem, also questioned the commission's claims that southern Delta state had a 98 percent turnout.

Observers and journalists saw little voting in the region, where Ijaw militants boycotted the vote to protest electoral boundaries that they said favored their ethnic rivals, the Itsekiris. The Ijaws are the largest tribe in the delta with eight million people.

Last month, fighting pitting Ijaws against Itsekiri and government troops took more than 100 lives and shut down 40 percent of Nigeria's oil production.

More than 10,000 people have been killed in ethnic, religious and political violence in Nigeria since Obasanjo's election four years ago ended more than 15 years of brutal military rule.

PHOTO CAPTION

People gather around a badly damaged bus that was involved in an accident and then attacked by a mob, in Kano, Nigeria, Thursday April 17, 2003. Nigeria's independent electoral commission is investigating the results of weekend legislative elections after political parties accused each other of vote rigging. (AP Photo/Schalk van Zuydam)