An election tribunal on Tuesday dismissed the main opposition party's challenge over fraud claims in the April presidential election, revalidating the ruling party's win in Africa's most populous nation.

The Congress for Progressive Change's election lawsuit failed to cast reasonable doubt on the results that handed victory to President Goodluck Jonathan about six months ago, said Judge Kumai Akaas, who led a panel of four judges that reached an unanimous decision.

"The petitioner did not discharge the burden of proof, even on the balance of probability," Akaas said.

Opposition candidate Muhammadu Buhari's party challenged the results of the April 16 vote soon after the nation's election body announced that Jonathan had won 22.4 million votes. The election body said Buhari had come in second place with 12.2 million votes, with the results giving Jonathan enough votes in at least 24 of Nigeria's 36 states to avoid a runoff.

The opposition vowed to appeal the ruling.

"We have judgment of conscience, judgment of people and judgment of God," opposition party chairman Tony Momoh said.

Leading to the vote, the opposition party enjoyed support in the country's predominantly Muslim north where a wave of postelection riots later left at least 500 dead and more than 40,000 people displaced.

Nigeria has a long history of violent and rigged polls since it abandoned a revolving door of military rulers and embraced democracy 12 years ago.

Observers said the April presidential polls appeared to be largely fair, with fewer cases of ballot box thefts than previous polls. However, irregularities still occurred and violence erupted across Nigeria's north after Jonathan's victory was announced, leaving hundreds dead.

Jonathan took office last year after the country's elected Muslim president died from a lengthy illness before his term ended. Many in the north believe the ruling party should have put up a Muslim candidate instead Jonathan, a Christian, for the election.