Buhari also warned against any attempts to tamper with the election, now scheduled to take place this coming Saturday. He said anyone trying to steal or destroy ballot boxes and voting material would be dealt with firmly. "I have given the military and the police instructions to be ruthless. We are not going to be blamed for the bad conduct of the election," he told an emergency meeting of senior members of his All Progressives Congress (APC) party in the capital Abuja.
The opposition has suggested that Buhari, a former military ruler who was later elected president in 2015, was behind the postponement in order to hold on to power. But all sides have appealed for calm in a country where past elections have been marred by violence and intimidation.
The Independent National Electoral Commission (INEC) announced the delay in the early hours of Saturday, just as some of Nigeria's 84 million registered voters were already making their way to polling stations. It cited logistical difficulties and problems with transporting election material to far-flung or conflicted areas, and denied any political pressure had been brought to bear.
"Definitely the reasons why such incompetence manifests itself has to be explained to the nation. After the election we have to know exactly what really happened," Buhari said, promising that an investigation would be launched. The electoral commission also said that campaigning would be suspended until the new voting day on Saturday.
But APC chairman Adams Oshiomhole told the meeting the party would resume its campaign despite the commission's order. Commission chairman Mahmood Yakubu Oshiomhole had erred in halting electioneering, Oshiomhole said. "INEC is not in a position to make laws. The law says you are entitled to renew campaigns up to 24 hours before the beginning of election," he said.
The opposition PDP also said it would get back on the campaign trail. It rejected the commission decision, which a PDP statement said was "directly in conflict with the provision of the Electoral Act and is not backed by any other law in our country". Buhari faces a tight contest against the PDP's Atiku Abubakar to lead a country that has Africa's largest economy and is its top oil producer but is plagued by corruption and wide gaps between rich and poor.
The election has been fought over Buhari's handling of the economy amid fragile growth and growing unemployment as the country recovers from a recession in 2016. Rising insecurity, notably in regions under attack by Boko Haram and other Islamist militants, is also a big worry. Buhari said anyone trying to interfere or intimidate voters would face a tough response.
"I want to warn anybody who thinks he has enough influence in his locality to lead a body of thugs to snatch boxes or disarm the voting system, he will do it at the expense of his own life," the president said. (Additional reporting by Paul Carsten, Aaron Ross, Camillus Eboh; Writing by James Macharia and Alexis Akwagyiram;)