ABUJA, Nigeria — A Nigerian election tribunal sided yesterday with the president in a challenge of the vote that brought him to power last year and that opposition leaders say was tainted by fraud.

The two opposition leaders vowed to appeal — sticking to a legal, peaceful route that is perhaps a sign of the African nation’s maturing democracy in contrast to its past and violence that followed Kenya’s disputed presidential election.

In a judgment delivered over three hours to a packed, sweltering courtroom, the tribunal said the petitions by former strongman Muhammadu Buhari and ex-Vice President Atiku Abubakar lacked evidence that fraud had materially affected the outcome of the April 21 vote.

President Umaru Yar’Adua welcomed the decision.

“Today’s event is a victory for Nigeria’s democracy and for the rule of law. On such occasion one must feel greatly humbled and also feel a great desire to recommit to the service of this great country,” he said in a statement. The two opposition leaders have 21 days to file their appeal before the highest court in Africa’s most populous nation.

“We’re going to the Supreme Court,” Buhari said.

The 2007 election saw power transferred from one elected civilian to another for the first time in Nigeria’s coup-plagued history. But thugs openly stole and stuffed ballot boxes and harassed voters, and international observers said the election was deeply flawed.

The election tribunal, however, requires proof not only that graft occurred, but that it was widespread enough to cause a different outcome.

Many Nigerians say the election demonstrated how weak the rule of law is in the nation of 140 million people. Still, the dispute over the vote has been peaceful and confined to the courts. That’s in contrast to Kenya, where violence after the Dec. 27 election has left more 1,000 dead and forced 600,000 from their homes.

In Nigeria, there were no widespread riots or protests after last year’s vote, although some believe that could still change.