Muhammadu Buhari set for victory in Nigeria, 6.40pm (GMT+1)

The All Progressives Congress candidate and  a former military ruler, Gen Muhammadu Buhari is ahead by almost three million votes.

In all terms of logic, it is impossible for the incumbent,  President  Goodluck  Jonathan to  overcome this lead.

The opposition has never won a presidential election in Nigeria, Africa's most populous state.

Observers have generally praised the election but there have been allegations of fraud, which could lead to protests and violence.

If confirmed, it would be the first democratic change of power in the history of Africa's most populous country and cap a remarkable return for the 72-year-old who headed a military regime in the 1980s.

With 34 out of 37 results in, the APC had won 19 states, while Jonathan's Peoples Democratic Party (PDP) was on 14, plus the Federal Capital Territory of Abuja.

"This is the first time the opposition has voted a government out of power in Nigeria's history," said APC spokesman Lai Mohammed.

Buhari was more than 2.75 million votes ahead of 57-year-old Jonathan, after winning Yobe and Adamawa, which have been among the hardest hit by the bloody, six-year Boko Haram insurgency.

The retired army general won the key prize of Lagos in the southwest but at one point his lead was cut to 500,000 votes after landslide victories for Jonathan in his southern Delta homeland.

The vote was the closest election contest ever in Africa's biggest economy which has a population of 173 million.

The opposition leader, making his fourth run at the presidency, has been buoyed by frustration over endemic corruption, criticism over Jonathan's handling of Boko Haram and a stronger opposition.

There was a brief protest by Jonathan's PDP before the count resumed on Tuesday.

Former Niger Delta minister Godsday Orubebe accused elections chief Attahiru Jega of being "partial" and "selective".

More than 800 people were killed in protests after Mr Jonathan beat Gen Buhari in the previous election.