* President Buhari, opposition's Atiku leading contenders
* Nigeria is Africa's biggest economy, top oil producer
* Blasts heard in city of Maiduguri - Reuters witnesses
* GRAPHIC-Nigeria presidential election: https://tmsnrt.rs/2E6qkDO (Adds analyst, voter's and military quotes)
By James Macharia and Ahmed Kingimi
ABUJA/MAIDUGURI, Feb 23 (Reuters) - President Muhammadu Buhari and his main challenger, businessman Atiku Abubakar, cast their ballots in Nigeria's presidential election on Saturday as voting began after a week's delay in Africa's biggest economic power.
Analysts say the vote is too close to call, with the outcome set to hinge on which man voters most trust to revamp an economy still struggling to recover from a 2016 recession.
Buhari, a former military ruler who was later elected president, is seeking a second term in charge of Africa's most populous nation and top crude producer. Atiku, a former vice president, has pledged to expand the role of the private sector.
The two candidates lead a field of more than 70 challengers.
Reuters witnesses observed some delays to the opening of polling stations and problems with voting systems in the early part of the ballot.
Last Saturday, the election was postponed around five hours before polling stations were due to open by electoral commission, which cited logistical factors.
Buhari, who voted in his hometown of Daura in the northern state of Katsina, said: "I will congratulate myself, I'm going to be the winner" when asked by reporters if he would congratulate his rival, should Atiku win the election.
Atiku later cast his ballot in the eastern Adamawa state.
"I look forward to a successful transition," he told reporters shortly after voting.
Nigerians queued at polling booths around the country where voting officially began at 8:00 a.m. local time (0700 GMT) and is due to close at 2:00 p.m.
However, several polling stations across the country were slow to open and others reported problems with machines meant to verify voters cards, Reuters witnesses said. The country has 72.8 million eligible voters.
"I've been to 10 polling units today. I've been redirected many times," said Victor Kanoba a voter in Lagos.
John Tomaszewski, an observer with the joint U.S. National Democratic Institute and International Republican Institute delegation, said delays had been somewhat expected given the logistical challenge of getting materials to the polling stations in time for opening.
"This will be something to watch throughout the day," he said.
"Logistics weren't properly managed despite the postponement of the polls," said Idayat Hassan, director of Abuja-based think tank Centre for Democracy and Development, which is also observing the election.
In the capital Abuja, Chukwunwike Ogbuani, a lawyer, said he was worried by the delays.
"This polling booth they say has about 20,000 registered voters... if there is at least up to 50 to 60 percent turnout it will be difficult to finish in a day. I don't see everybody that is here voting within the stipulated time."
In Lagos' business district of Victoria Island, Reginald Anthony, 45, who runs a transport business, said: "We are seeing a transparent election, everything is open for everyone to see".
After voting in the northern Kano state, Hadisa Hayatu, a 38-year old housewife, said: "I voted for Buhari because he has assured us that he is going to build on what he has done on security and other issues."
An Atiku supporter in Kano, stylist Laurie Isaac, 27, said: "We need change. I need more work. I need my salary to increase."
In the country's northeast, where insurgent groups like Boko Haram and Islamic State West Africa Province have waged a decade-long war, blasts were heard in the city of Maiduguri, the capital of Borno state, said Reuters witnesses. Boko Haram had warned people not to vote.
Meanwhile, residents in the village of Geidam in Yobe state, which neighbours Borno, said they fled an attack by suspected militants.
"We have along with our wives and children and hundreds of others fled. We are right now running and hiding in the bushes," said Geidam resident Ibrahim Gobi, speaking by phone.
Colonel Sagir Musa, acting director of army public relations, said there had not been an attack on any part of Maiduguri, where activity had been part of an exercise by the military.
Musa said there was an attack on a security outpost in Geidam but there were no casualties.
Security sources told Reuters that militants had struck at parts of the city and a Reuters witness said he had heard gun shots and Nigerian air force jets were flying overhead.
(Reporting by Paul Carsten, Seun Sanni, Aaron Ross, Abraham Achirga, Adewale Kolawole, Afolabi Sotunde, Ardo Hazzad, Didi Akinyelure, Garba Muhammed, Mike Oboh, Nneka Chile, Ola Lanre, Percy Dabang, Camillus Eboh, Christian Merenini and Tife Owolabi; Writing by James Macharia; Editing by Alexis Akwagyiram and Toby Chopra)
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