LAGOS (Reuters) - Nigeria's main opposition coalition started a convention on Wednesday to select a candidate to take on President Goodluck Jonathan in elections next February that are expected to be the closest fought since the end of military rule in 1999.
Two political heavyweights led the five-strong field for the opposition All Progressives Congress (APC) ticket -- former military ruler Muhammadu Buhari and ex-vice president Atiku Abubakar. Both men are Muslims from the country's north.
Jonathan, a southern Christian, has come under fire for a mounting insurgency by Boko Haram Islamists and faces a tough re-election battle. Underlining the country's security woes, a suicide bomber killed four in Kano on Wednesday.
Eight thousand delegates began gathering in Nigeria's national stadium in the commercial capital Lagos, to vote for their choice. Results are expected on Thursday.
Buhari is the frontrunner. He has huge grassroots support and is seen as one of the few leaders in the country's history who was tough on corruption after he seized power in a 1983 coup. He was deposed himself less than two years later.
"A respectable man like General Buhari, I'm sure he can win," said Femi Afolabi, a retired journalist from the southwest. "He's upright, disciplined and against corruption."
Corruption remains a lightening rod for anger in Nigeria, where many perceive it as the root cause of the country's poverty and dysfunction. Jonathan's administration has been implicated in several oil corruption scandals, some of which it has denied, while others it has promised to investigate.
Lagos traffic, bad at any time, ground to a standstill. Outside the stadium supporters beat drums and cheered, some taking refuge from the tropical humidity under palm trees.
The ruling People's Democratic Party (PDP) holds its primary on Thursday, but it is a formality since Jonathan has already been approved by its board as the sole candidate.
The Feb. 14 election will test whether Africa's biggest economy and top oil producer can learn from past mistakes that have often seen votes marred by fraud and violence. Campaigning will occur against a backdrop of plunging fiscal revenues and a currency devaluation triggered by falling oil prices.
The likely contest between an opposition candidate from the largely Muslim north and an incumbent from the mostly Christian south sets up a regional and sectarian divide that could be a flashpoint for trouble.
Jonathan's bid for a second elected term has upset northern elites, who argue he broke an unwritten deal that power rotate between north and south every two terms. He ran in 2011, after replacing northerner Umaru Yar'Adua, who died in office in 2009.
Northern anger has also been fueled by growing perceptions of a shift in power toward the more prosperous south. More than 800 people were killed and 65,000 displaced in three days of rioting in the north after Jonathan's 2011 win against Buhari.
Abubakar has largely campaigned on the achievements of his government under ex-president Olusegun Obasanjo, which include getting Nigeria's debt written off and creating Economic and Financial Crimes Commission (EFCC) to fight graft and fraud.
Reporting by Tim Cocks; Editing by Andrew Heavens and Crispian Balmer