Nigeria's electoral watchdog on Saturday postponed presidential and parliamentary elections for one week, just hours before polls were due to open. Polling had been due to start at nearly 120,000 polling units in Africa's most populous nation at 0700 GMT, with a record 73 candidates on the ballot. President Muhammadu Buhari is seeking a second term of office, but is facing a stiff challenge from the main opposition candidate, former vice-president Atiku Abubakar.