- Reacting to the nomination, the presidency congratulated Abubakar.
- The politician and business tycoon has made four previous bids for the top job in Nigeria, Africa's most populous nation.
- Buhari, the retired general who headed a military regime in the 1980s, has faced growing pressure to step down because of failing health.
- Fierce Buhari critic Obiageli Ezekwesili announced that she would throw her hat into the ring for the presidency.
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 People's Democratic Party (PDP) in Port Harcourt, nominated Abubakar, 71, as his challenger for next year's poll.
Reacting to the nomination, the presidency congratulated Abubakar but added that it "noted with interest all the reports in the media as to massive vote-buying at the PDP primaries".
"One wonders what such a candidate would do with public funds," the statement added.
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.
Despite humble beginnings in northern Nigeria, he rose through the ranks of the customs service for two decades before entering the private sector, investing in oil services and agriculture, among others.
From there he joined the civilian government where he became one of Nigeria's most recognisable and enduring politicians.