A Nigerian tribunal will uphold or annul the 2007 election of President Umaru Yar'Adua on Tuesday in a ruling that could either legitimise a deeply flawed poll or push Africa's most populous nation into political limbo.

Yar'Adua and his People's Democratic Party (PDP) won a crushing victory in last April's general elections but local and international observers accused the ruling party of massive fraud. EU monitors judged the polls "not credible".

Yar'Adua's two nearest rivals, former army ruler Muhammadu Buhari and then Vice-President Atiku Abubakar, challenged the result before a special tribunal of five judges which is due to start delivering its ruling at 10 a.m. (9. a.m British time).

Police cordoned off the area around the courthouse to keep out chanting supporters of the challengers. A few ministers and PDP state governors arrived to show their support for Yar'Adua.

The tribunal faces a dilemma. It can legitimise an election that was labelled "a charade" by the main Nigerian observer group, or risk destabilising a major oil exporter that emerged just nine years ago from decades of coups and army rule.

Some commentators saw the tribunal ruling as a positive development regardless of the outcome because, unlike in Kenya where the disputed December 27 poll led to violence on the streets, Nigeria's aggrieved candidates took their protests to court.

"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.

Whatever the tribunal's decision on Tuesday, the legal battle is likely to drag on because the losing parties can appeal to the Supreme Court.

If the judgment went against Yar'Adua and he appealed, he would remain president until the Supreme Court delivers its verdict. However, he would be weakened and government business would likely grind to a halt until the final ruling.

Many in the political class fear annulment would lead to a long period of political gridlock followed by an equally flawed re-run under the same electoral body.

Lower tribunals, at state level, have already annulled the elections of seven out of 36 state governors, the Senate president and dozens of federal and state legislators.

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