Britain and the United States have said there are signs that the final count for Nigeria's presidential election is being subverted by "deliberate political interference".
A joint statement says the countries would be "very concerned" by any attempts to undermine the independence of the electoral commission and distort the will of the Nigerian people.
It comes as the start of the collation of votes was delayed without explanation from a scheduled noon local time start today.
The statement came just before the electoral commission began the official collation of votes from 36 states and the Federal Capital Territory of Abuja, in the presence of party representatives, national and international observers and media. The adding up of votes from all over Nigeria began about two hours later than scheduled.
For the first time in Nigeria's history, a presidential vote appears too close to call, analysts have said of the high-stakes election.
The race has come down to a contest between president Goodluck Jonathan and former military dictator General Muhammadu Buhari.
Widespread rigging has marred many previous Nigerian elections and efforts to make it more difficult with new biometric cards have been spoiled in part by some failures of newly imported card readers.
"So far, we have seen no evidence of systemic manipulation of the process. But there are disturbing indications that the collation process - where the votes are finally counted - may be subject to deliberate political interference," said a statement signed by US secretary of state John Kerry and his British counterpart, Philip Hammond.
"The governments of the United States and the United Kingdom would be very concerned by any attempts to undermine the independence of the Electoral Commission (INEC), or its chairman, Prof. (Attahiru) Jega; or in any way distort the expressed will of the Nigerian people," the statement said.
There was no immediate comment from Nigeria's ruling party or government.
Nigerians are waiting in hope and fear for results of a presidential election in which voting was relatively smooth nationwide despite technical glitches, deadly attacks by Islamic extremists and allegations of political thuggery in some areas.
The winner of the vote in Africa's most populous and richest nation could be announced late today or tomorrow, electoral officials said.
While there are fears that violence could erupt once results are announced, one radio station played a reconciliation song written by entertainment star 2Face Idibia in Nigeria's colloquial English: "Vote not fight! Election no be war!"
Police in Port Harcourt, Nigeria's oil capital, fired tear gas to disperse thousands of women supporters of the opposition coalition who demanded the cancellation of the election in Rivers state.
The opposition is demanding new elections in the southern states of Rivers and Akwa Ibom, alleging irregularities that include missing and false results sheets and electoral officials being replaced by government officials loyal to Mr Jonathan. The national election commission said it is investigating numerous complaints.
Observers for the African Union said that the weekend voting was conducted in a peaceful atmosphere and "largely meets the continental and regional principles of democratic elections".
Mr Jonathan, 57, is a Christian from a minority tribe in the lush oil-producing south and 72-year-old Mr Buhari is a Muslim from the semi-arid north that is home to farmers, cattle herders and centuries-old caliphates.
After Mr Buhari lost to Mr Jonathan in 2011, more than 1,000 people died and some 65,000 were forced from their homes in northern riots, according to the National Human Rights Commission.
There was a high turnout among the nearly 60 million people who had cards to vote in this election, which for the first time offers the possibility of a challenger defeating a sitting president.
Voters are also electing legislators for parliament, where the opposition currently has a slight edge over Mr Jonathan's party in the House of Assembly. Voting for 13 constituencies was postponed until April because of shortages of ballot papers.
Dozens of legislators defected from Mr Jonathan's party to a new opposition coalition that has transformed Nigeria's political landscape by uniting behind one candidate for the first time.
Detractors accuse Mr Jonathan of being ineffectual and Mr Buhari of being anti-democratic.
Mr Jonathan's party has governed since decades of military dictatorship ended in 1999.
Some progress has been made - a nearly 20% boost in manufacturing and a mini agricultural revolution tripling production of some basics such as rice - but corruption is endemic and the vast majority of Nigeria's 170 million people are poor.
Just days before the elections, Nigeria's military, backed by armies from neighbouring countries, announced major victories over home-grown Boko Haram Islamic extremists after months of defeats.
On the political front, supporters of both sides are threatening violence if their candidate does not win, with militants in the Niger Delta saying there will be "all-out war" if Mr Jonathan loses.