Nigeria's election began with gunfire and explosions as President Muhammadu Buhari seeks a second term.Delays at polling stations across the West Africa country have also marred the election which is widely seen as too close to call.Security authorities acknowledged an extremist attack in the northeast city of Maiduguri shortly before voting began on Saturday.A statement by Borno police said Boko Haram extremists "attempted to infiltrate" the state capital by launching artillery fire in a bid to disrupt the elections.The statement said "some missiles strayed into vulnerable locations" but there were no casualties and the extremists retreated.However, according to security sources, one soldier was killed and four wounded.They had earlier said the reported blasts had been their own in a show of force to deter extremists.Image:President Muhammadu Buhari casts his voteGunfire was also heard in Port Harcourt in Nigeria's restive south where the presence of the army was heavier than in previous elections.:: Your guide to the Nigerian electionA military convoy in Delta state contained more than 25 vehicles full of soldiers on standby.Soldiers in Rivers state fired on suspected ballot snatchers and arrested four people.Mr Buhari refused to answer questions on whether he would accept a loss to top challenger Atiku Abubakar, a billionaire former vice president.Image:Opposition presidential candidate Atiku Abubakar pictured votingAs he went out to vote in his northern hometown of Daura, the president jokingly checked the name on his wife's ballot.Nigerians "are behaving themselves", the president said.After voting in his hometown of Yola in the north east, Mr Abubakar said: "I look forward to a successful transition".He previously pledged to accept the results provided they are credible.Image:The election is widely seen as too close to callMr Buhari said the voting process had been smooth but a coalition of civic groups said many polling stations had not opened more than four hours after voting was due to officially start.Delays were reported in Delta, Anambra and Akwa Ibom states as well as in Nigeria's largest city Lagos.Votes are now being counted and observers say preliminary results of the election are expected within two to four days.