KANO, Nigeria — Nigerian President Muhammadu Buhari surged to an early lead in election returns Monday, winning seven of 36 states in Africa's largest democracy, while the main opposition rejected the count, alleging manipulation. Election observers said the last-minute postponement of the vote until Saturday discouraged some Nigerians from going to the polls as Buhari seeks a second term heading a country troubled by corruption, insecurity and a weak economy. Buhari, a former military dictator from the north, faced a strong challenge from top opposition candidate Atiku Abubakar, a former vice president and businessman, in a race many saw as too close to call. As of midnight, Abubakar had won three southern states and the territory that includes the capital, Abuja. Buhari led by more than 450,000 votes overall. Abubakar's party chairman, Uche Secondus, accused ruling party agents of hacking into the electoral commission's computer server and manipulating results...