The declaration yesterday that Mr Obasanjo, a 63-year-old retired general, had won the race drew immediate claims of ballot-stuffing from his opponent, Olu Falae.
By last night, General Obasanjo had captured 61 per cent of the vote after 29 of Nigeria's 36 states and the federal capital had declared their results. Mr Falae disputed the results, which showed him more than 5 million votes behind, after 25 million ballot papers had been counted.
"If General Obasanjo won a free and fair election I would congratulate him but clearly this is not a free and fair election," Mr Falae said.
Western observers also reported voting malpractices in many areas though they mostly concluded they were insufficient to have significantly altered the result, which ends 15 years of military rule.
"There were some disparities noted by ourselves," said the former American president Jimmy Carter, who jointly leads an American delegation. "Some of the local officials apparently permitted exaggerated reports of voter participation and in some cases there were some ballots in the box that were not cast by voters."
The election was one of the final elements in a transition programme to democracy that began last October and has included local, gubernatorial and parliamentary polls. But the questions raised over the result puts a strain on efforts to lead Africa's most populous country away from military rule.
Foreign approval for the election is essential for Nigeria. The country is trying to regain world respect after years of human rights abuses and corruption and it desperately needs foreign financing after a collapse in world oil prices.
The transition programme is spearheaded by General Abdulsalami Abubakar. He became head of state after the death last June of General Sani Abacha, a tyrannical dictator who executed his opponents and pocketed the nation's oil wealth. Under General Abubakar's plan, President Obasanjo will take the reins on 29 May.
The views of election observers are unlikely to halt the progress of the transition programme. Over the next three months a constitution must be agreed that is acceptable to the powerful military.
General Obasanjo will also withdraw Nigeria's troops from the West African intervention force in Sierra Leone.
The winning candidate, who is widely respected in the West for having handed over power to a civilian when he was last head of state 20 years ago, must also instigate talks with other pro-democracy groups. These will be pressing for a more power to be devolved to federal level.
General Obasanjo faces the task of both working with the military, and of sending them back to the barracks. At the same time, he must satisfy restive campaigners for democratic change.
Both General Obasanjo and Mr Falae belong to the Yoruba ethnic group. The need to appease Yoruba sensibilities was behind the decision to put up candidates from their region in the election, which marks a shift in power from the conservative, Muslim north.Reuse content