ABUJA, Nigeria -- In a cliffhanger of an election, early returns Monday night from half the states showed Nigerian President Goodluck Jonathan and former military dictator Muhammadu Buhari almost even. The U.S. and Britain warned of "disturbing indications" that the tally could be subject to political interference.
Counting stopped just before midnight Monday with Jonathan winning nine states and the tiny Federal Capital Territory to Buhari's nine states.
But Buhari won many more votes - 8.5 million to Jonathan's 6.48.
Another 18 states - including Lagos which has the biggest number of voters of any state - still have to send results to the counting center in Abuja, electoral commissioner Attahiru Jega announced. He said the count resumes at 10 a.m. (0900 GMT) Tuesday.
It's the first time in Nigeria's history that a challenger has a real chance of defeating a sitting president. This is only the eighth election since independence from Britain in 1960.
Buhari swept the northern states of Kano and Kaduna, as expected, but delivered unexpectedly crushing defeats to Jonathan. In Kano, the state with the second-largest number of voters, Buhari won 1.9 million votes to Jonathan's 216,000. In Kaduna, Buhari won 1.1 million votes to Jonathan's 484,000.
Protests erupted Monday in two southwestern states won by Buhari's party. In one of them, Osun, Channels TV showed video of the electoral official who raced through the announcement and then dashed out of the hall followed by other scrambling officials and media with security personnel rushing to protect them from the anticipated fallout.
Authorities declared a curfew in southern Rivers State after unofficial results not yet announced by the electoral commission gave Jonathan 95 percent of votes there.
The count in Abuja was being carried out in the presence of party representatives, national and international observers and media. The counting began two hours late, with no explanation given for the delay.
The U.S. and Britain issued a joint statement saying they would be "very concerned" by any attempts to undermine the independence of the electoral commission and distort the will of the Nigerian people.
"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 the statement, signed by U.S. Secretary of State John Kerry and his British counterpart, Philip Hammond.
Jonathan's party called the suggestions "absolute balderdash" and demanded evidence. Campaign spokesman Femi Fani-Kayode told foreign reporters that they "completely reject the assertion or the notion that we are in any way interfering with INEC," the electoral commission.
"I will challenge John Kerry or any other foreign official to provide the evidence," he said.
Widespread rigging has occurred in many previous elections, along with violence after those votes. New biometric cards aimed at stemming fraud were used but some newly imported card readers were not working properly, and voting was extended to Sunday in 300 out of 150,000 polling stations where that problem occurred, the election commission said.
Turnout was high Saturday among the nearly 60 million people eligible to vote in the high-stakes election, which took place despite a campaign of violence by the Islamic extremists of Boko Haram in northern Nigeria.
Jonathan and Buhari are the front-runners among 14 candidates for president.
The National Human Rights Commission said 50 people were killed during the balloting, including a state legislator, a soldier and two electoral workers.
Nevertheless, commission chairman Chidi Odinkalu said the election showed "a maturing political system."
"The best guarantee of a violence-free election is a credible count and collation," Odinkalu said in a statement.
Relatively smooth voting was reported in this nation of 170 million people despite technical glitches, deadly attacks by Boko Haram, and allegations of political violence and threats in some areas. There was still concern, however, that the announcement of the results could trigger violence.
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 opposition supporters who demanded the cancellation of the election in Rivers state.
The opposition wants 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 was investigating numerous complaints.
Just days before the vote, Nigeria's military, backed by armies from neighboring countries, announced major victories over Boko Haram extremists after months of defeats.
The U.N. secretary-general's special envoy to West Africa, Mohamed Ibn Chambas, told the Security Council 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.
Nigeria's deputy ambassador to the U.N., Usman Sarki, said he hoped the Security Council can vote on a resolution this week that would endorse military action by a five-country regional force against Boko Haram. He and colleagues were waiting for instructions from their capital on issues that include whether the resolution should invoke Chapter 7 of the U.N. Charter, meaning it could be militarily enforced. Nigeria is opposed to Chapter 7, which gives the U.N. the final say, and wants a Chapter 8 mission that leaves it in the driving seat, according to diplomats in Abuja who have discussed the matter with Nigerian officials.
Jonathan, 57, is a Christian from a minority tribe in the lush oil-producing south, while 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.