Nigerian opposition leader Muhammadu Buhari built a lead of 2 million votes with three-quarters of states counted on Monday, raising the prospect of a stunning ballot box victory for him.
Leaders of the All Progressives Congress say ballot rigging and other problems occurred in Anambra and Akwa Ibom states.
Nigerian President Goodluck Jonathan's campaign representative has accused the media of spreading false results in favor of the opposition party.
The country's presidential and parliamentary elections continued for a second day, after Boko Haram attacks and claims of voting irregularities.
“At least half the village has been burned," a federal lawmaker from Buratai says.
The presidential candidates, challenger Muhammadu Buhari and incumbent Goodluck Jonathan, were in a dead heat before the election.
There are 14 presidential candidates on the ballot, but national polls have shown the race will come down to President Goodluck Jonathan and rival Muhammadu Buhari.
Nigerian authorities have increased security across the nation, including closing all land and sea borders until after the election.
The Boko Haram headquarters in Gwoza, Nigeria, had been one of the few strongholds it had left in northern Nigeria.
Some voters see ex-military dictator Muhammadu Buhari as a solution to Nigeria's security and corruption issues. But there are concerns about his credentials.
“Without a stronger effort to protect civilians and accountability for abuses, the situation can only get worse," a researcher said.
Security concerns range from political violence to attacks by Boko Haram.
Allegations of ballot rigging is a dangerous narrative in Nigeria’s presidential elections, and Jonathan is no stranger to such accusations.
With elections looming, the U.S. president calls on Nigeria’s leaders and citizens to ensure they are free, fair and peaceful.
Chadian strikes killed several dozen Boko Haram fighters in the Nigerian village of Djaboullam, said a senior Niger official.
Victims of the terror group's "killing and maiming spree" are discovered as international troops retake lost Nigerian territory.