Early results in Nigeria's closely contested election showed former military ruler Muhammadu Buhari leading incumbent President Goodluck Jonathan, election officials said late Monday.
Buhari won 10 states, including several in Jonathan's traditional stronghold in the south, according to the Independent National Electoral Commission. Jonathan netted three states and the federal capital territory, Abuja.
As officials continuing tallying the millions of votes from this weekend's national elections, questions emerged about the counting process and tensions rose in some states about possible vote-rigging.
Independent observers said the balloting, which was extended by a day after technical problems arose with a new anti-fraud system, was conducted without major problems.
But concerns arose that the process of counting the raw votes may be in danger of being manipulated.
The Nigerian Civil Society Situation Room, one of several local groups monitoring the polls, said politicians might use security agencies to “fiddle with the election collation process” at the state level.
In an indication of the potential seriousness of the problem, U.S. Secretary of State John Kerry and British Foreign Secretary Philip Hammond released a joint statement Monday saying there were "disturbing indications" that the collation process "may be subject to deliberate political interference."
Jonathan, a Christian from the south, has been in office since 2010 and his Peoples Democratic Party has held the presidency since 1999. Buhari, a Muslim from the north, is heading up the ticket for the opposition All Progressives Congress.
To win the election without a runoff, a candidate needs a majority of the nationwide vote while also securing at least 25 percent of the ballots in two-thirds of the country's 36 states and the capital.
Delays, attacks at polling sites
The vote was scheduled to be held on Saturday, but technical glitches with a new biometric vote system led officials to extend voting into Sunday. Some polling stations, particularly in the north, were hit with attacks from Boko Haram militants.
Observer missions by the African Union and the Economic Community of West African States said, despite logistical problems and some violence, the organization of the vote was "acceptable."
The National Democratic Institute, a nongovernmental organization funded in part from the U.S. and other governments, praised the election, despite the technical problems and sporadic violence.
Christopher Fomunyoh, a Nigerian expert with the organization, said Monday the group saw no evidence of the military interfering in the poll, as many had feared.
However, “we have concerns about the collation process because of the multilayered collation mechanism," Fomunyoh said.
That referred to the fact that results are collated separately in each polling station and state as well as in the main center in Abuja, a practice that should be reviewed given the new technology, he said.
The statement by Kerry and Hammond echoed those concerns.
The U.S. and British governments "would be very concerned by any attempts to undermine the independence of the electoral commission or its chairman ... or in any way distort the expressed will of the Nigerian people," the statement said.
Festus Okoye, national coordinator for the Independent Election Monitoring Group and a member of the Situation Room, said there were concerns in three states in particular: Rivers, Imo and Benue.
“Now to collate the results, people are trying to tamper with it," Okoye said.
"We believe this election represents a quantum leap in our democratic aspirations and in our determination to institutionalize democracy in this country," he said. "We will not like a situation where some elements within the political elite who did not fight for democracy will take us back to where we are coming from and we will not tolerate that type of situation.”
In the southern state of Rivers, tensions mounted as opposition officials and supporters called for the election to be re-done. The state's governor reportedly refused to cast his vote over concerns the local vote could be manipulated.
Police on Monday used tear gas to disperse hundreds of women dressed in black, who marched on the state electoral office in Port Harcourt.
The country's top election official, Attahiru Jega, said his office was investigating the complaints in Rivers and other states.
"There are many alleged cases of malpractices and we certainly pay a lot of attention to investigating this and if our staff are involved in any way or manner, obviously we will apply the appropriate sanctions and take the appropriate decisions as provided for by the electoral legal framework,” Jega said.
Elsewhere, the military fired warning shots after hundreds of youth gathered outside the local electoral office in Bauchi state, shouting opposition slogans and shouting that they would protect their vote from rigging.
Nigeria was originally due to hold the election in mid-February, but officials pushed back the vote because of fighting and instability in northeastern states where Boko Haram militants have been battling the government since 2009.
The security situation improved after a multinational offensive drove the militants from many towns they had controlled.
Also contributed were Ibrahim Yakubu in Kaduna, Hilary Uguru in Port Harcourt, Ardo Hazzad in Bauchi, Chris Stein in Kano and Katarina Hoije in Lagos. Some material for this report came from Reuters and AP.