ABUJA - A Nigerian tribunal upheld the 2007 election of President Umaru Yar'Adua on Tuesday, rejecting challenges from rivals who wanted the vote annulled because of massive rigging.

The tribunal in Africa's most populous nation ruled that opposition candidate Muhammadu Buhari, Yar'Adua's main opponent, had failed to prove that violations of the electoral law were substantial enough to invalidate the overall result.

The five-judge tribunal also rejected a challenge brought by former Vice-President Atiku Abubakar, who came third in the election that local and international observers said was marred by massive fraud.

"Umaru Yar'Adua and Goodluck Jonathan remain the president and vice-president of the Federal Republic of Nigeria," the ruling said.

Had the vote been annulled, it could have risked destabilising the major oil exporter, which emerged just nine years ago from decades of army rule and has drawn increasing foreign investor interest.

Yar'Adua's victory was disputed from the moment it was announced and doubts over whether he would finish his four-year term have delayed policy-making and investment decisions.

Some commentators saw the simple fact that the challenge went through the courts as a positive development on a continent where disputed elections can lead to violence, as Kenya's December 27 vote has done.

"The ultimate winners are our democracy and indeed our judicial system, along with millions of Nigerians let down by the kleptomaniacs who shot or rigged their way into positions of leadership," wrote Muhammad Al-Ghazali in Daily Trust newspaper.

The legal battle in Nigeria is still likely to drag on because the losing parties can appeal to the Supreme Court.

Official results from the election on April 21 last year gave Yar'Adua 24.6 million votes, compared with 6.6 million for Buhari and 2.6 million for Abubakar.

A few months ago, most Nigerians would have considered it unthinkable for a court to overturn a presidential election, but multiple annulments of appointments by lower level appointments have changed that perception.