On Tuesday, a group of 18 opposition parties threatened to boycott the national election unless the government could guarantee "transparency and fairness".
But after three days of meetings, opposition parties were unable to reach a decision and the two main Nigerian opposition parties, the All Nigeria People's Party (ANPP) and the Action Congress (AC), announced separately on Thursday that they will take part in polls to find a new head of state and legislative assembly.
"How many elections on the African continent have ever been fair?"
Mack Rogers, Chattanooga, TN, USA
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The US, the EU and rights groups have expressed serious reservations over the polls and called on Nigeria's government to take immediate action to prevent electoral misconduct.
The election to choose a successor to Obasanjo is being billed as the first civilian-to-civilian handover since the world's sixth biggest oil exporter won independence from Britain in 1960.
Obasanjo admitted that the conduct of last week's election was "marred", but said electoral malpractice including "massive rigging" which has characterised past elections was on the decline.
"Let us continue to improve on the structure and the house rather than pull it down because it is leaking in part," he said.
"I call on INEC [Independent National Electoral Commission] do everything possible to strengthen the integrity of the process, the system and the outcome.
"There is no perfect society but other societies identify their weaknesses and shortcomings and collectively make correction by strengthening the positive and eliminating the negative."
He also urged aggrieved candidates and their supporters to "explore all avenues for seeking redress rather than taking to jungle justice and running down the totality of our country and all it stands for."