The election pits President Goodluck Jonathan, the first head of state from the southern, oil-producing Niger Delta, against Muhammadu Buhari, a former military ruler with a reputation as a disciplinarian from the mostly-Muslim north.

Other candidates include former anti-corruption chief Nuhu Ribadu and Kano state governor Ibrahim Shekarau, although they are seen as rank outsiders.

The African giant, home to more people than Russia, has failed to hold a free and fair presidential election since military rule ended in 1999, leaving many of its 150 million citizens with little faith in the benefits of democracy.

But a relatively successful parliamentary election a week ago, deemed credible by observers despite isolated acts of violence, has renewed confidence in the ability of the electoral commission, INEC, to make a break with the past.

Campaigns have also largely been free of inflammatory rhetoric so far, assuaging fears that a race between southern Christian Jonathan and northern Muslim rivals could polarize Nigeria, home to about equal numbers from both religions.

President Jonathan, a former zoology teacher born to a family of canoe makers, is the front-runner. He is backed by the national machinery of the People's Democratic Party (PDP), whose candidate has won every presidential race since 1999.

But he is resented by some in the north, who believe he is usurping the right of a northerner to the presidency for another four years. Jonathan inherited office after his predecessor, northerner Umaru Yar'Adua, died last year in his first term, interrupting a rotation between north and south.