Nigeria's main opposition party has asked a federal court to overturn results from the presidential election, calling the vote flawed though international observers say it was the cleanest in the fledging democracy's history.

Rioting and reprisal attacks following the April 16 vote won by President Goodluck Jonathan left at least 500 people dead and forced 40,000 to flee their homes across the country's north.

The party of runner-up Muhammadu Buhari filed its lawsuit Sunday, alleging that widespread fraud had allowed Jonathan's ruling party to cling to power.

"We want the tribunal to nullify elections in these areas where there were flaws and conduct fresh elections in those areas," said Tony Momoh, the chairman of the opposition party.

Buhari, who lost to Jonathan by more than 10 million votes, had said before the election that he would not got to court if he lost. The former military ruler also had contested in Nigeria's 2003 and 2007 elections, and unsuccessfully challenged the results each time. Buhari did say, however, that his party was free to oppose the results.

"Who will benefit if his party wins? said Thompson Ayodele, director of the Lagos-based Institute for Public Policy Analysis. "Indirectly, Buhari is the one who is going to court."

Nigeria's judicial system is accustomed to postelection lawsuits, with courts extending their hours in the one-month window between the polls and the swearing-in ceremony. The tradition stems from a history of openly rigged elections since Nigeria became a democracy in 1999. International observers though, said the April elections were the cleanest Nigeria has held in more than a decade.

"I don't see him winning the case either after the election was adjudged as widely free and fair, and secondly justice systems are always pro-establishment," said Ayodele. "I don't understand the party's reasoning. Maybe, they just want to make a point."

A Muslim from the north, Buhari enjoyed strong support in the region. When preliminary counts foreshadowed his defeat, violence spread quickly across northern states.

Many there believe that the 2011 presidency should have gone to a Muslim from the north as an unwritten agreement in the ruling party calls for its presidential candidates to rotate between the country's Christian south and Muslim north.

Jonathan, a Christian from the south, became president last year only after the death of the long-ailing Umaru Yar'Adua, a Muslim northerner who was not able to finish his term or seek re-election.


Associated Press writer Bashir Adigun contributed to the report from Abuja, Nigeria.