Nigeria's electoral commission began counting votes in hotly contested presidential elections Monday as the United States and Britain warned that the count may be subverted by "deliberate political interference."

For the first time in Nigeria's history, a presidential vote appears too close to call, analysts have said of Saturday's high-stakes election to govern Africa's richest and most populated nation. Front-runners are President Goodluck Jonathan and former military dictator Gen. Muhammadu Buhari.

The voting was relatively smooth in this nation of 170 million people despite technical glitches, deadly attacks by Islamic extremists and allegations of political violence and threats in some areas.

Widespread rigging has occurred in many previous Nigerian elections, along with post-election violence. New biometric cards aimed at stemming fraud were brought in but some newly imported card readers were not functioning properly. Voting was extended to Sunday in some 300 out of 150,000 polling stations where that problem occurred, the election commission said.

The winner could be announced late Monday or on Tuesday, electoral officials said.

A joint statement by the United States and Britain said both countries would be "very concerned" by any attempts to undermine the independence of the electoral commission and distort the will of the Nigerian people.

The official collation of votes from 36 states and the Federal Capital Territory of Abuja was to be carried out in the presence of party representatives, national and international election observers and media. There was a high turnout among the nearly 60 million people who had cards to vote in this election that for the first time offers the possibility of a challenger defeating a sitting president. The vote counting began two hours late with no explanation for the delay.

Nervousness that the announcement of the results could trigger violence was palpable. One radio station played a song written by entertainment star 2Face Idibia in Nigeria's colloquial English: "Vote not fight! Election no be war!"

After Buhari lost to 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.

Police in Port Harcourt, a center of oil production in Nigeria's south, fired tear gas Monday morning 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 Jonathan. The national election commission said it is investigating numerous complaints.

Just days before the elections, Nigeria's military, backed by armies from neighboring countries, announced major victories over home-grown Boko Haram Islamic extremists after months of defeats.

The U.N. secretary-general's special envoy to West Africa, Mohammed Ibn Chambas, told the Security Council in a briefing Monday that Boko Haram was "unable to disrupt the electoral process." The legitimacy of Nigeria's next government is important for ensuring the support of the international community, he said.

African members of the council said discussions continue on a draft resolution that would endorse military action by a five-country African force against Boko Haram. Issues that remain include funding of the force.

Jonathan, 57, is a Christian from a minority tribe in the lush oil-producing south and 72-year-old Buhari is a Muslim from the semi-arid north that is home to farmers, cattle herders and centuries-old caliphates.

Voters also are electing legislators for parliament, where the opposition currently has a slight edge over 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 Jonathan's party to a new opposition coalition that has transformed Nigeria's political landscape by uniting behind one candidate for the first time and face Jonathan, whose party has governed Nigeria since decades of military dictatorship ended in 1999.

Detractors accuse Jonathan of being ineffectual and Buhari of being anti-democratic.

Some progress has been made -- a nearly 20 percent boost in manufacturing and a mini agricultural revolution tripling production of some basics like rice -- but corruption is endemic and the vast majority of Nigerians remain poor.