Mr Obasanjo voted at ward 11, unit 22, Olusomi Compound, Abeokuta-North Local Government Area of Ogun State after a long delay due to late arrival of INEC officials.
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The former president had already declared his support for PDP presidential candidate, Atiku Abubakar.
Mr Obasanjo was an ally of President Muhammadu Buhari, whom he supported for the 2015 general elections.
He has, however, accused the president of nepotism, poor performance, and sectionalism and declared his support for Mr Abubakar.