"I'm voting for continuity", Ruka Ajana, a voter, told NPR.
Mrs Buhari got cleared to vote before her husband but Buhari was seen looking at her ballot before he proceeded into the booth to cast his ballot.
Addressing the nation on Friday morning, President of Nigeria Muhammadu Buhari said: "Do not be afraid of rumours of violence and unrest".
Buhari was elected president in 2015, but many Nigerians believe he has failed to live up to his initial campaign promises which included cracking down on rampant corruption and fighting Boko Haram.
In an emergency meeting on Monday, President of Nigeria Muhammadu Buhari was widely criticised for saying that anyone who interfered in elections "will do it at the expense of his own life".
That could affect participation in some affected areas and combine with voter apathy from last weekend's postponement to affect turnout, according to analysts.
"The postponement casts a lingering doubt on the neutrality of (the electoral commission) such that unless Atiku is declared the victor, many will still believe that (the commission) colluded with the government to rig him out", said Jideofor Adibe, associate professor of political science at Nasarawa State University.
Officials have already begun counting votes, though most observers say victory is too close to call, with the outcome set to hinge on which man voters most trust to revamp an economy still struggling to recover from a 2016 recession.
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"But where they are not resolved before the closing of elections, the election will continue the next day".
Governor Ibrahim Gaidam of Yobe State has been prevented from going to cast his vote by Boko Haram terrorists.
In southern Rivers state, unknown gunmen shot dead a prominent local member of Buhari's All Progressives Congress party and his brother, police said. He previously pledged to accept the results, provided they are credible.
"They are late and it's not good because I wanted to vote on time and go home", said Akinola Kogo, 57, as he waited for registration to start at a Lagos primary school.
The blasts in the Borno state capital of Maiduguri came shortly before polls were to open.
There was no official comment from the military but Borno police commissioner Damian Chukwu denied any attack.
Gunfire also has been heard in parts of Port Harcourt in the restive south, where the military presence is said to be heavier than in past elections.
But the Boko Haram extremist group, its Islamic State-affiliated offshoot in the northeast and various agitators across the country, including bandits, oil militants and youth hired by politicians to disrupt the vote, could have other plans.