The peace accord ceremony was held days after the PDP said authorities had frozen the bank accounts of its vice presidential candidate, Peter Obi.

Elections to choose the leader of Africa’s most populous country – the continent’s top oil producer and by many measures its largest economy – have in the past been marred by violence, vote-rigging and voter intimidation.

The ceremony was an attempt to mirror the signing of an acclaimed deal ahead of voting in 2015, when Buhari came to power. It was credited with helping prevent a repeat of violence in the wake of the 2011 election which Human Rights Watch said killed 800 people and displaced 65,000.

The accord aims to ensure that the 2019 elections are “free, fair and credible,” the document said. Signatories pledged that their campaigns would refrain from “religious incitement” and “ethnic or tribal profiling”, and that they would not incite violence, according to the document.

The PDP said it was investigating the circumstances behind the “unfortunate communication lapse” between party officials and the National Peace Committee (NPC), the non-governmental organisation that arranged the event, which resulted in Abubakar’s absence.

“Our presidential candidate Atiku Abubakar and the party are fully committed to a peaceful electoral process and elections in 2019,” said PDP spokesman Kola Ologbondiyan in an emailed statement.

He said the PDP would visit the NPC office to “fulfil all necessary obligations toward the peace accord ahead of the 2019 general elections”.

The NPC earlier said it had sent invitations for the event to all 91 registered parties and the 73 presidential candidates.

Independent observers for the 2015 race, the first time the agreement was signed, have since praised the accord for helping Nigeria pull off its first-ever peaceful transition from the ruling party to the opposition. Buhari defeated the incumbent Goodluck Jonathan.