After two days of voting, incumbent President Goodluck Jonathan is reportedly neck-and-neck with his challenger and former military ruler Muhammadu Buhari. US and UK officials have expressed concern over vote rigging.
Nigeria's election commission was expected to release the results of the highly-contentious presidential race between late on Monday evening and Tuesday morning.
Unofficial results gave President Jonathan and his People's Democratic Party (PDP) three states, as well as the Federal Capital Territory.
Opponent Buhari - who hails from Nigeria's Muslim north - and his All Progressives Congress (APC) reportedly won five states.
News agency DPA indicated that the rivaling parties had won roughly the same number of votes, with PDP leading at 2,322,734 over APC's 2,302,978.
In order to win the presidency and avoid a run-off vote, one candidate must win the most votes and clinch at least 25 percent of support from two-thirds of Nigeria's 36 states.
Conditions were reportedly calm on Monday evening in the northern state of Kano, where post-election violence claimed the lives of roughly 1,000 people in 2011.
A government spokesperson told AFP news agency that there was "record turnout" among Nigeria's nearly 60 million registered voters. Over two days of voting, which were extended due to voting equipment malfunctions, Nigerians also cast ballots for 109 Senate seats and 306 seats in the National Assembly.
Concerns of vote rigging
In a joint statement issued on Monday, US Secretary of State John Kerry and his British counterpart, Philip Hammond, expressed concern over election conduct.
"So far, we have seen no evidence of systematic manipulation of the process," the two chief diplomats said.
"But there are disturbing indications that the collation process - where the votes are finally counted - may be subject to deliberate political interference."
Following the polls, there were reports of technical glitches and late delivery of election materials.
However, Nigeria's Transition Monitoring Group said: "These issues did not systematically disadvantage any candidate or party."
EU observers also said that polls were conducted fairly.
Meanwhile, a group of 2,000 female protesters took to the streets of the southern city of Port Harcourt on Monday in a demonstration against alleged election misconduct. Police responded with tear gas.
Boko Haram didn't impact vote
The United Nations also confirmed on Monday that Islamist militant group Boko Haram - whose headquarters the Nigerian army recently captured - had been "unable to disrupt the electoral process" over the weekend, despite attacks in northeastern Bauchi state.
Since the beginning of 2014, the violent insurgency has claimed some 7,300 lives, the UN further reported.
President Jonathan's popularity has suffered under the Islamist insurgency, which has expanded its campaign from terrorist attacks to mass kidnappings and cross-border assaults over the past year.
kms/sb (AP, AFP, Reuters, dpa)