Nigerians will head to the polls on Feb. 25 to select a successor to Buhari, who cannot run because the constitution does not permit him to serve more than two consecutive terms.
Veteran Bola Tinubu from Buhari's ruling All Progressives Congress party, main opposition People's Democratic Party's Atiku Abubakar and Peter Obi from the smaller Labour Party are the main contenders.
Past elections have been marred by deadly violence and intimidation, ballot snatching, vote buying and allegations of rigging, which have dented the credentials of a country that emerged from decades of military rule in 1999.
Buhari told a regional election monitoring mission that Nigerians should freely choose their next leader.
"We shall not allow anyone to use money and thugs to intimidate the people, said Buhari, without saying how this would be stopped.
"Nigerians know better now, they are wiser, and know that it is better to dialogue than to carry weapons. Elections are even more difficult to rig now."
Campaigning has so far been largely peaceful. Voters will use biometrics to cast their ballots and results will be transmitted in real time from polling stations, which the electoral commission hopes this will prevent rigging.
(Reporting by Felix Onuah; writing by MacDonald Dzirutwe; Editing by Angus MacSwan)