Goodluck Jonathan is set for election as Nigeria's president with almost double the vote of his main rival, partial results suggest.
Figures provided by regional officials suggest he has garnered enough votes to avoid a run-off.
A BBC correspondent says there is a sense of relief and jubilation that the vote and count have been relatively calm, unlike in past years.
However, some results in individual states have been suspiciously high.
Mr Jonathan had staked his reputation on the election, repeatedly promising it would be free and fair.
Unless the national electoral commission declares a large chunk of the votes to be invalid, he is now on track to become Nigeria's first elected president from the oil-producing Niger Delta region.
The Christian politician was appointed to the presidency last year, upon the death of incumbent Umaru Yar'Adua, whom he had served as vice-president.
To win at the first round, a candidate needs at least 25% of the vote in two-thirds of Nigeria's 36 states.
According to regional results, Mr Jonathan has passed that threshold in at least 24 states. He polled 20.3m votes to the 10.4m cast for his nearest rival, General Muhammadu Buhari.
In Akwa Ibom state, he was credited with winning 95% percent and in Anambra it was 99%. In his home state, Bayelsa, he took 99.63%.
"Figures of 95% and above for one party suggest that these are fabricated figures and, personally, they worry me because they pose serious questions on the credibility of the election," Jibrin Ibrahim of the Centre for Democracy and Development told AFP news agency.
Former government minister Nasir el-Rufai, a supporter of Gen Buhari, told Reuters: "In most of the south-east and south-south, no real elections took place.
"In the south-west and the north, the results have no relation to what happened at the polling units and we will prove it in due course."
A spokesman for the general, Yinka Odumakin, also said irregularities had taken place but any challenge would come after the vote count.
Mr Jonathan's campaign team have said they will not publicly comment until the election commission has formally declared all the results in the capital Abuja, an announcement expected later on Monday.
"This is no time for triumphalism," Oronto Douglas, a senior adviser to Mr Jonathan, told Reuters news agency.
"It is a time for deep reflection, for strengthening the bond of our union and for all of us to work together."
Previous polls were marred by widespread violence and vote-fixing but Saturday's election was reported to have generally gone smoothly, after violence in the run-up left dozens of people dead.
A woman is said to have been killed in the central city of Jos on polling day and two bomb explosions hit the north-eastern city of Maiduguri.
Casting his vote in Bayelsa, Niger Delta, Mr Jonathan said the election was a "new dawn in Nigeria's political evolution".
"Nigeria is now experiencing true democracy where we the politicians have to go to the people," he added.
In Daura, home to Gen Buhari, crowds waited for hours despite the intense heat to cast their votes.
The general predicted the ruling party would try to rig the election in a "sophisticated way", he told the BBC.
But he added that he had more faith in the national electoral commission this time round, adding that "probably 60% of the election" would be credible.
Other challengers for the presidency included former anti-corruption chief Nuhu Ribadu and Kano state governor Ibrahim Shekarau.