Nigeria's President Muhammadu Buhari speaks a launch campaign for his re-election, in Uyo. File picture: Tife Owolabi/Reuters.

Lagos - Nigerian opposition candidate Atiku Abubakar on Friday called President Muhammadu Buhari's suspension of the country's top judge "an act of dictatorship", just weeks from elections.

Abubakar, of the Peoples Democratic Party, said the decision was a "brazen dictatorial act" and "the latest action in the ongoing rape of our nation's hard-earned democracy".

Buhari earlier announced that Chief Justice Walter Onnoghen would be suspended pending the conclusion of a trial on charges of failing to declare foreign currency bank accounts.

The president's opponents have accused him of trying to manipulate the judiciary, which would rule on any dispute in the forthcoming elections on February 16.

Abubakar, who is seen as Buhari's main challenger, has previously accused the ruling All Progressives Congress of trying to rig the result.

Buhari defended the suspension, saying the government was "dissatisfied with the alarming rate in which the Supreme Court of Nigeria under... Onnoghen has serially set free persons accused of the most dire acts of corruption, often on mere technicalities" after they had been convicted.

But Abubakar said the suspension was "an anti-democratic act" and "geared towards affecting the outcome of the 2019 presidential elections".

He added: "This brazen dictatorial act is the latest action in the ongoing rape of our nation's hard-earned democracy by those who dined with anti-democratic forces".

The decision is "symptomatic of the increasing desperation that President Buhari and the cabal pulling the strings have as February 16, 2019 draws near", he said. 

Onnoghen on Thursday secured a Court of Appeal injunction ordering the lower Code of Conduct Tribunal to halt the case, pending a ruling on his application to dismiss the charges.

Abubakar questioned why due process was not allowed to run its course and called for the international community to threaten "strong consequences" for anyone involved.

The United States and Britain have said anyone involved in vote-rigging or election violence would be denied visas.