President Muhammadu Buhari and Atiku Abubakar joined other candidates to sign a "peace accord", in front of foreign observers, including Commonwealth head Patricia Scotland.

The signatories called on their supporters "to refrain from violence or any acts that may in any way jeopardise our collective vision of a free, fair and credible election".

"We also pledge to respect the outcome of free, fair and credible elections," they added.

Some 84 million people are registered to vote on Saturday in presidential and parliamentary elections. Polling for new state governors and state assembly members is on March 2.

Buhari came to power in 2015 as the first opposition candidate to defeat a sitting president. In another first, his opponent, Goodluck Jonathan, also conceded defeat.

The head of state said this year's vote was another landmark, with more women and young people, and a record 91 parties and more than 70 presidential candidates.

But he called for greater engagement in politics to help address pressing issues in Africa's most populous nation.

"Elections by themselves do not constitute democracy," said Buhari, of the ruling All Progressives Congress (APC) party.

"It is the spirit of citizenship engagement that utilises the energy of citizens towards the attainment of good governance."

Abubakar, a former vice-president representing the Peoples Democratic Party (PDP), called on the election commission and security agencies to be neutral.

Claims that Buhari and the APC is looking to rig the result has been a feature of the PDP campaign, as has reminding voters of Buhari's past as a former military ruler.

He said in a speech that "General Buhari" needed to ensure that "every eligible person who casts their vote is confident in the process and in the belief that their vote will count".

Buhari, who he has accused of "dictatorship", also needed to "abide by the will of the people as freely expressed through elections, under the terms of our constitution", he added.

This year's ceremony was organised by former military ruler Abdulsalami Abubakar and his National Peace Committee, and backed by the Kofi Annan Foundation.

Former US president Bill Clinton was invited but decided not to travel to Abuja because his presence had "the potential to be politicised", his spokesman, Angel Urena said.