ABUJA (Reuters) - President Muhammadu Buhari took an early lead on Monday in initial official results from Nigeria’s presidential election, but the party of his main challenger quickly rejected the tallies as “incorrect and unacceptable”.

Saturday’s election, which U.S. observers said had lost some credibility after being abruptly delayed by a week by officials citing organizational glitches, was expected to be Nigeria’s tightest since the end of military rule two decades ago.

At stake is control of Africa’s top oil producer and biggest economy. Northeast Nigeria has also been wracked by a decade-long battle with Islamist militants that has spilled into neighboring countries and led to the deployment of a regional task force.

Buhari, 76, is a former military ruler seeking a second term on an anti-corruption platform, while Atiku Abubakar, 72, a businessman and former vice president, has pledged to expand the role of the private sector.

Initial results released by the Independent National Electoral Commission (INEC) on Monday put Buhari in the lead, having won in seven of Nigeria’s 36 states, the commission said.

Atiku prevailed in four states and the capital, Abuja, which is not a state but treated as a separate district in elections.

Buhari led Atiku by 51 percent to 46 percent, according to a Reuters tally based on the figures given by the commission.

In provisional results announced in state capitals but not yet confirmed by the commission, Buhari had won four states to Atiku’s one.

In the northern state of Kano, hundreds of people celebrated Buhari’s provisional lead in what was billed as one of the key battlegrounds in this year’s elections.

“Kano is for Buhari,” Wahab Abdulawal, a trader, said.

Asked about the early results, Buhari told reporters: “I don’t want to depend on rumors. ... We will rather wait for INEC to announce the (full) results.”

But the chairman of Atiku’s People’s Democratic Party (PDP), Uche Secondus, said the election tallies announced so far were “incorrect and unacceptable”.

“The results are being manipulated and canceled for APC to retain power,” he told reporters, referring to Buhari’s party.

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Buhari’s All Progressives Congress (APC) said in a statement that the PDP had “embarked on a cynical plan to discredit INEC as a backup plan in the likely event of them losing the election”.

The outcome, expected this week, appears to hinge on which man voters trust most to revamp an economy still struggling from a 2016 recession.

A credible and relatively calm vote would open a new chapter in the checkered political history of Nigeria, where nearly six decades of independence have been tarnished by military coups, endemic corruption and secessionist movements.

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But doubts rose when the election was postponed on Feb. 16, just hours before it was due to begin, with authorities citing problems in delivering ballot papers and results sheets.

‘NEGATIVE EFFECTS’

The week-long delay in holding Nigeria’s presidential election damaged public trust in the process and probably reduced Saturday’s voter turnout, U.S. observers said.

The civil society group YIAGA AFRICA, which monitored the election, projected turnout at 36 to 40 percent.

Situation Room, a monitoring mission comprising over 70 civic groups, said on Sunday that as many as 39 people had been killed in election-related violence, and more than 260 in all since the start of the campaign in October.

Voting, however, took place “in a generally peaceful environment”, said Hailemariam Desalegn, head of the African Union observer mission and a former premier of Ethiopia.

“There were scattered incidents of violence but it was not seen as pervasive on Election Day,” said Derek Mitchell, president of the U.S. observer mission.

Previous Nigerian elections have been marred by violence among supporters of different political parties that at times sparked conflict between Christians and Muslims. Security forces are currently stretched by the Islamist insurgency as well as by communal violence and banditry in other areas.

Hours before polls opened, explosions rocked Maiduguri, capital of Borno state, epicenter of the insurgency. In neighboring Yobe, residents of the town of Geidam fled a militant attack around the same time.

Scattered violence and problems with smart-card readers that authenticate voters’ fingerprints meant voting in a small number of precincts was put off to Sunday, Mitchell said.

“Serious operational shortcomings put an undue burden on voters,” Maria Arena, the EU’s chief observer and member of the European parliament, told reporters.