* Close contest between President Buhari and rival Atiku
* Buhari wins 4 of first 5 districts counted -official tally
* Results “incorrect, unacceptable” -main rival’s party
* Dozens killed in election-related violence
* Election delay eroded trust, hit turnout -U.S. observers (Recasts with Buhari in early lead, opposition challenge)
By Camillus Eboh and Felix Onuah
ABUJA, Feb 25 (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 organisational glitches, was expected to be Nigeria’s tightest since the end of military rule two decades ago.
At stake is the helm of Africa’s top oil producer and biggest economy, rattled by a decade-long battle with Islamist militants in the northeast that has spilled into neighbouring countries and prompted 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, 72, a businessman and ex-vice president, has pledged above all to expand the role of the private sector.
Initial results released by the national electoral commission (INEC) on Monday put Buhari in the lead. He won in four of Nigeria’s 36 states – two in the southwest, and one each in the east and central parts of the country, INEC said, while Atiku prevailed in the capital Abuja.
In provisional results announced in state capitals but not yet confirmed by INEC, Buhari took the northeastern state of Yobe while Atiku won in Ondo state in the southwest.
Asked about the early results, Buhari told reporters: “I don’t want to depend on rumours…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 cancelled for APC to retain power,” he told reporters, referring to Buhari’s party.
The full outcome, not expected before later in the 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 chequered political history of Nigeria, where nearly six decades of independence have been tarnished by military coups, endemic corruption and secessionist movements.
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.
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.
“It is highly probable that such a late postponement had negative effects on voter turnout,” John Tomaszewski of the joint U.S. National Democratic Institute (NDI) and International Republican Institute observer delegation, told reporters.
“More significantly, the delay also undermined public confidence in INEC,” he said.
The civil society group YIAGA AFRICA, which monitored the election, projected turnout at 36-40 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 over 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 sectarian conflict. 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, epicentre of the insurgency. In neighbouring 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 had to 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.
Additional reporting by Alexis Akwagyiram and Paul Carsten
Writing by James Macharia
Editing by Mark Heinrich