Muhammadu Buhari was on Wednesday re-elected Nigeria’s president, after a delayed poll that angered voters and raised political temperatures, leading to claims of rigging and collusion.

With ballots counted in all of Nigeria’s 36 states and the Federal Capital Territory (FCT), Buhari, 76, triumphed with some 15.2 million votes over his nearest rival Atiku Abubakar, who trailed by nearly four million votes.

“Muhammadu Buhari… is hereby declared winner and is returned elected,” Mahmood Yakubu, chairperson of Nigeria’s Independent National Electoral Commission (INEC) announced early Wednesday.

It was the second victory at the ballot box for Buhari, a one-time military ruler who in 2015 was elected to lead Africa’s most-populous nation and leading oil producer.

Addressing supporters and party leaders at his All Progressives Congress (APC) campaign headquarters, he called his win “another victory for Nigerian democracy.”

“The new administration will intensify its efforts in security, restructuring the economy and fighting corruption,” he said. “We will strive to strengthen our unity and inclusiveness so that no section or group will feel left behind, or left out.”

Supporters had gathered outside the party’s offices in the capital Abuja late Tuesday as it became clear that Buhari had an unassailable lead, dancing and singing “We’re popping champagne!”

Vice-President Yemi Osinbajo appeared in a video clip singing “Up we go!” in a reference to Buhari and his “Next Level” campaign slogan.

But there were none of the spontaneous street parties that marked his victory four years ago, when he became Nigeria’s first opposition candidate to beat an incumbent president.

To win the presidency, a candidate needs a majority of votes nationwide and at least 25% of support in two-thirds of the states plus the FCT.

The results showed Buhari won 15 191 847 votes (56%) while Abubakar, of the Peoples Democratic Party (PDP) won 11 262 978 votes (41%).

Buhari won in 19 states – including the two most populous, Lagos and Kano – while Abubakar was victorious in 17 states and the FCT.

There was no immediate official concession from Abubakar, whose party had earlier called on INEC to halt the count, citing irregularities.

In 2015, president Goodluck Jonathan won plaudits for conceding to Buhari in a phone call, when the results indicated he could not win.

The concession is acknowledged as having prevented a repeat of the political violence that has marked previous elections in Nigeria, in which hundreds of people have been killed.

This year’s election — the sixth in the 20 years since the return of civilian rule after decades of military government was initially scheduled for February 16.

But INEC postponed them for one week just hours before polls were due to open, citing logistical difficulties in delivering ballot boxes and other election materials.

Voting took place on Saturday after a week in which the APC and PDP intensified their war of words, accusing the other of conspiring with INEC to rig the result.

The PDP’s claimed data from handheld devices used to authenticate voters’ identities at nearly 120 000 polling units had been altered in favour of the ruling party.

It called for the results in at least two states to be re-run and for a reversal of the cancellation of tens of thousands of “valid, legal” votes elsewhere.

Observers also pointed out discrepancies in declared tallies.

The claims and Abubakar’s insistence that he would only accept a free, fair and credible election could be a prelude to a legal challenge of the outcome.

The vote was marred by violence, including 53 deaths, according to the Situation Room, an umbrella group of more than 70 civil society groups that monitored polling.

Some observers reported instances of vote-buying, intimidation and violence towards voters and officials, which have been a problem in previous polls in Nigeria.

The issues will likely prompt calls for electoral reform, including the introduction of technology capable of directly transmitting results from polling units.