Nigerian President Muhammadu Buhari waves to the crowd during the 58th anniversary celebrations of Nigerian independence, in Abuja, Nigeria on Monday. Picture: AP Photo/Olamikan Gbemiga

Nigerian President Muhammadu Buhari will run for re-election in February 2019 against former vice president Atiku Abubakar, a Muslim from the country's north who was nominated on Sunday as the main opposition party's poll contender.

Buhari, a 75-year-old former military ruler, was the sole candidate for his ruling All Progressives Congress (APC) party, so his confirmation by some 7,000 delegates gathered in the capital Abuja was a mere formality on Saturday.

The APC swept to power in 2015 with the first opposition victory at the ballot box in the country's history.

But next year's presidential race appears to have tightened in recent months with the APC hit by a wave of defections over Buhari's leadership style.

On Sunday, delegates to a convention of the former ruling Peoples Democratic Party (PDP), nominated Abubakar, 71, as his challenger for next year's poll.

The politician and business tycoon has made four previous bids for the top job in Nigeria, Africa's most populous nation.

Abubakar comes from the Muslim-majority north, and his nomination follows an unwritten rule in Nigeria that the presidency should alternate every two terms between a candidate from the north and south.

The PDP vote was held in the oil hub of Port Harcourt, in the heartland of the southern Niger delta.

Abubakar garnered 1,532 votes, trouncing his closest rival Aminu Tambuwal, the governor of northern Sokoto state who scored 693 votes.

Port Harcourt streets had been festooned with conference banners, while police said thousands of officers were deployed to the PDP primary.

Despite humble beginnings in northern Nigeria, Abubakar rose through the ranks of the customs service for two decades, eventually becoming the institution's number two during military rule.

He then entered the private sector, investing in oil services and agriculture, among other industries.

From there he joined the civilian government where he became one of Nigeria's most recognisable and enduring politicians.

But he has been dogged by controversies over his numerous wives and more than 20 children as well as corruption allegations.

Buhari had indicated in April that he planned to run for a second term.

The retired general, who headed a military regime in the 1980s, has faced growing pressure to step down because of failing health after spending several months in London last year receiving treatment for an undisclosed ailment.

Dubbed "Baba Go Slow" because he took six months to appoint cabinet ministers, he has also faced attacks for his handling of the economy, which plunged into recession in 2016.

He has also come under criticism on security issues, including the Boko Haram insurgency in the northeast, long-running farmer-herder clashes in the centre and militancy and kidnapping in the south. 

"The fact that nobody came forward to challenge the president is an indication that the party members are satisfied with his performance and they want to see more of him in office," presidential spokesman Garba Shehu told AFP.

Also Sunday, fierce Buhari critic Obiageli Ezekwesili announced that she would throw her hat into the ring for the presidency.

The 55-year-old former minister, who was also the World Bank’s vice president for Africa from 2007 to 2012, said in a statement that Nigeria was becoming "a country where the worth of life is trending down to zero."

Ezekwesili, co-founder of the Berlin-based corruption watchdog Transparency International, is best known in Nigeria for creating the BringBackOurGirls movement after Boko Haram jihadists abducted more than 200 schoolgirls in 2014.

Also in the running is Donald Duke, the former governor of the southern Cross River state, whose opposition Social Democratic Party formally elected him its flag-bearer on Sunday.

As governor in 2004, Duke initiated the Calabar carnival that is now popularly known as "Africa's biggest street party."

Nigerian law allows for a president to serve a maximum of two four-year terms.

Voters in the former British colony will elect governors and lawmakers as well as the president in elections set for February and March next year.