The European Union says the president's decision "raises concerns about opportunity for electoral justice." The suspended chief justice would play a role in any dispute following the upcoming presidential vote.
The EU Election Observation Mission (EU EOM) said it was "very concerned" about Nigerian President Muhammadu Buhari's decision to suspend the country's chief justice just weeks before a presidential election.
The mission, which is set to observe the presidential vote, said the timing of the suspension "raises concerns about opportunity for electoral justice."
Read more: 2019 Nigeria election: What you need to know
Buhari, who was a military ruler in the 1980s and elected to office in 2015, is seeking a second term in the February 16 election.
The judiciary and the chief justice would play a major role in the event of a disputed election. Past elections in the oil-rich and notoriously corrupt country have been marred by irregularities and violence.
Financial misconduct charges
The suspension of Chief Justice Walter Nkanu Samuel Onnoghen on Friday came as he was preparing to swear in members for election petition tribunals.
Onnoghen faces trial for allegedly failing to declare foreign currency assets as required of officials under Nigerian law. The allegations surfaced two weeks ago.
In addition, "the security agencies have since then traced other suspicious transactions running into millions of dollars to the CJN's personal accounts, all undeclared or improperly declared as required by law," Buhari said in a statement on Friday. CJN refers to the chief justice of Nigeria.
On Thursday, an appeals court had issued an interim order to stop charges being brought against the chief justice.
However, Buhari accused the courts of "stalling of the trial" under the influence of Onnoghen and his lawyers' tactical moves.
He was therefore forced to suspend Onnoghen because the judge didn't voluntarily step down and swear in Justice Ibrahim Tanko Muhammed as Nigeria's most senior judge, Buhari said. He said he was acting on the directive of the Code of Conduct Tribunal.
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.
Muhammed, like the president, is from Nigeria's largely Muslim north while Onnoghen is from the predominantely Christian south. Buhari has been accused of stacking his government with people from the north.
'Brazen dictatorial act'
Buhari's top election challenger, former Vice President Atiku Abubakar, accused Buhari of attempting to affect the outcome of the presidential election.
"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 and is symptomatic of the increasing desperation that President Buhari and the cabal pulling the strings have as February 16, 2019 draws near," he said in a statement.
The Nigerian Bar Association condemned an "attempted coup" against the judiciary and suspension of the constitution by the president.
"The action of the Executive portends a slide into anarchy and complete deconstruction of the rule of law and due process. It amounts to an absolute breach of the constitution and the usurpation of the powers of the Senate and the Nigerian Judicial Council," it said in a statement.
The United States and Britain have also said anyone involved in vote interference and violence would face consequences, including visa restrictions.
amp,cw/cmk (AFP, AP, Reuters)