Nigerian opposition leader Muhammadu Buhari built a lead of two million votes with three-quarters of states counted today.
This raised the prospect of a stunning ballot box victory for a man who first came to power three decades ago via a military coup.
Mr Buhari recorded majorities in key northern states, as the United States and Britain expressed concerns about meddling with the vote count.
Mr Buhari, a 72-year-old former military ruler, who has campaigned as a born-again democrat intent on cleaning up corrupt politics, won 1.1 million votes in the flashpoint city of Kaduna.
President Goodluck Jonathan, 57, won 484,000 votes there.
The city, scene of three days of violence after Mr Buhari lost to Mr Jonathan in the last election in 2011, was tense but quiet as the results trickled in, with the roads empty of traffic and many shops and homes shuttered.
Mr Buhari also won 1.9 million votes in Kano against 216,000 for Mr Jonathan, an indication of the political polarisation that has deepened over the last five years under People's Democratic Party (PDP).
Although the economy has been growing at 7% or more, scandals over billions of dollars in missing oil receipts and the eruption of an Islamist insurgency in which thousands have died have undermined Mr Jonathan's popularity.
Although the early results will hearten the Buhari camp,they are far from conclusive in an election forecast to be the closest since the end of military rule in 1999.
In Rivers state, in his home Niger Delta region, the hotly contested home of Africa's biggest oil and gas industry, Mr Jonathan won a massive 95% of the vote.
The results coming from states such as Rivers have prompted suspicion among diplomats, observers and the opposition, whose sympathisers took to the streets in protest.
Police fired tear gas at a crowd of 100 female supporters of Mr Buhari's All Progressives Congress (APC) demonstrating outside the regional offices of the INEC election commission.
The weekend vote was marred by confusion, technical glitches, arguments and occasional violence but in many places proved to be less chaotic than previous elections in Africa's biggest economy and top oil producer.
At least 15 people were shot dead on polling day, most of them in the northeast where the islamist insurgent group Boko Haram has declared war on democracy in its fight to revive a mediaeval caliphate in the sands of the southern Sahara.
However, the United States and Britain said that after the vote there were worrying signs of political interference in the centralised tallying of the results.
"So far, we have seen no evidence of systemic manipulation of the process," US Secretary of State John Kerry and British Foreign Secretary Philip Hammond said in a joint statement.
"But there are disturbing indications that the collation process - where the votes are finally counted - may be subject to deliberate political interference," they added.
