Officials declared early Wednesday that Muhammadu Buhari had won a second term as president of Africa’s most populous country, where voters rejected a corruption-stained candidate in favor of a leader who promised to continue a campaign to eliminate graft.

The 76-year-old defeated his main rival, former Vice President Atiku Abubakar, with a margin of nearly four million votes.

Mr Abubakar’s People’s Democratic Party (PDP) has rejected the result. Turnout was 35.6%.

Delays and violence marred the run-up to Saturday’s poll but no independent observer has cited electoral fraud.

Mr Buhari’s All Progressives Congress (APC) won in 19 of the 36 states while the PDP was victorious in 17 states and in the capital, Abuja, according to the electoral commission (Inec).