Nigeria's President Muhammadu Buhari on Sunday defended his record on the economy and the outcome of a disputed presidential election, saying he was leaving a legacy of credible and fair votes, a day before he hands over power to his successor.

Buhari first came to office in 2015 after promising to reboot the economy and end corruption and insecurity, but many Nigerians say these issues have worsened under his watch.

Incoming President Bola Tinubu's victory is being challenged by his two closest opposition rivals, and on Tuesday a tribunal will begin to hear the main arguments in the election petition.

Buhari, an ascetic 80-year-old retired general, said the February vote had helped entrench democracy in Africa's most populous nation and that Tinubu, who ran on his ruling party's ticket, was the best candidate to emerge from the election.

"I am leaving behind an electoral process which guarantees that votes count, results are credible, elections are fair and transparent and the influence of money in politics reduced to the barest minimum," Buhari said in a farewell national broadcast.

Tinubu is inheriting anemic economic growth, record debt and shrinking oil output. Double-digit inflation, which has eroded savings and wages, is one of the biggest issues that will confront him when he is sworn into office.

But Buhari said his government had made some difficult choices to reset the economy, some of which "led to temporary pain and suffering for which I sincerely apologized to my fellow countrymen, but the measures were taken for the overall good of the country."

Life is tough for Nigerians, and a tangle of protectionist economic policies and foreign currency interventions have caused dollar shortages and spooked investors.