Nigeria's main opposition presidential candidate has filed a court petition to nullify the re-election of President Olusegun Obasanjo on the grounds of widespread vote rigging and other irregularities. Muhammadu Buhari, who came a distant second to Obasanjo in the 19 April ballot, filed his claims on Tuesday at the Court of Appeal in Abuja. The court serves as a tribunal to consider complaints arising from the elections. No date has yet been fixed for a hearing.