Nigeria's oppostion has chosen former military dictator Muhammadu Buhari to challenge President Goodluck Jonathan at next year's presidential elections, according to results released from party primaries.

Buhari, a Muslim northerner who overthrew a democratically elected president in 1983 and then himself was deposed in a coup in 1985, handily won primaries of a four-party opposition coalition, according to Thursday's results.

The February 14 vote is expected to be the most closely contested since decades of military rule ended in 1999 in Africa's most populous nation and continent's biggest oil producer.

Jonathan was the sole candidate at the governing People's Democratic Party primaries though his election early on Thursday flouted an unwritten party rule that the presidency should rotate between a Christian southerner, like himself, and a northern Muslim.

Dozens of ruling party legislators have defected over the issue, costing Jonathan's party its majority in the lower house of parliament.

But Jonathan told party faithful on Thursday that "today we are stronger, bigger, and more in tune with the yearnings of our people".

The opposition accuses Jonathan, 57, of failing to contain an uprising that has killed thousands and driven 1.2 million people from their homes.

Buhari, 71, will be contesting presidential elections for a fourth time and as a former army general, he is touted as more likely to succeed in the fight against Boko Haram.

He also is praised for fighting corruption while he was in power.

Alluding to that in a speech to party delegates, Buhari said "I am not a rich person. I can't give you a fistful of dollars or naira to purchase your support."

He accused Jonathan of incompetence, saying "instead of resolving problems, this government multiplies and manufactures them".

Jonathan is accused of fuelling corruption in an administration that has confronted one multimillion-dollar scandal after another, reported the Associated Press news agency.

In his acceptance speech, Jonathan said Nigeria's GDP has grown from $35.9bn when his party came to power in 1999 to $510bn today.

Source: Agencies