Buhari said the allegations against Onnoghen were "grievous enough".
Nigerian President Muhammadu Buhari meets with U.S. Secretary of State Mike Pompeo (not pictured) on the sidelines of the United Nations General Assembly in New York City, U.S. September 28, 2018.
Mr Onnoghen was removed as he prepared to inaugurate members of the election tribunal.
The suspension of Justice Walter Onnoghen and swearing in of Justice Tanko Mohammad as acting CJN stirred widespread controversy and attracted condemnations both locally and from the worldwide community. Buhari's election in 2015 was a rare peaceful transfer of power. And if that is the case, then such a person is not fit to hold the exalted position of Chief Justice of Nigeria.
The decision came after Onnoghen was put on trial last week by the Abuja-based Code of Conduct Tribunal for an allegedly false declaration of his assets.
Using similar language, the Nigerian Bar Association - the umbrella body for lawyers - said Buhari had mounted an "attempted coup against the Nigerian judiciary".
Under Nigerian law, the chief justice would only have to step down if convicted or if the Senate upholds a presidential request to do so by a two-thirds majority.
PDP's candidate, businessman Atiku Abubakar who served as vice president from 1999 to 2007, is the main challenger to Buhari in the upcoming poll.
Abubakar questioned why due process was not allowed to run its course and called for the global community to threaten "strong consequences" for anyone involved.
While responding to the suspension remarks through a telephone interview, All Progressive Congress spokesman Lanre Issa -Onilu said they were optimistic of a win and downplayed the suspension of campaigns by PDP terming it as an "excuse" for its defeat.
Onnoghen has since criticised what he said was the politicisation of judicial appointments in Nigeria and cleared Senate leader Bukola Saraki on corruption charges.
Buhari has been accused of surrounding himself with ministers, advisors and officials from the predominantly Muslim north, his home region.
He added, "The alleged suspension of Justice Walter Onnoghen from office of the CJN is the vilest, thieving, most despicable, ultra vires, undemocratic and brazenly unconstitutional act ever carried out by any government in Nigeria, civilian or military, since 1st January, 1914, when the contraption called Nigeria was forcibly contrived through the amalgamation of Northern and Southern Protectorates".