International pressure mounted on Nigeria's President Muhammadu Buhari on Saturday after he suspended the country's top judge just weeks away from elections, prompting claims from opponents and civil society groups of an attempted judicial "coup".The European Union, the United States and Britain expressed concern at the removal of Chief Justice Walter Onnoghen, who as head of the Supreme Court would have ruled on any dispute in the February 16 elections.Onnoghen is accused of failing to declare foreign currency bank accounts, in breach of rules governing public officials. But the timing of the charges and the manner of his removal have caused consternation.Former military ruler Buhari, 76, was accused of overreaching his powers for bypassing parliament, which constitutionally has to sanction such a move.Onnoghen had been on the verge of swearing in judges of election tribunals. His replacement, acting chief justice Ibrahim Muhammad Tanko, conducted the ceremony on Saturday.The European Union Election Observation Mission said it called on "all parties to follow the legal processes provided for in the constitution and to respond calmly to any concerns they may have".The US Embassy in Abuja said it was "deeply concerned" Onnoghen had been replaced "without the support of the legislative branch".The British High Commission in the capital voiced its "serious concern" and said the decision "risks affecting both domestic and international perceptions on the credibility of the forthcoming elections".London and Washington this week threatened consequences for anyone involved in electoral fraud or violence, including the refusal of visas.In a statement late on Saturday, the Nigerian presidency said it was determined to ensure free, fair and credible elections but would not allow foreign interference in the nation's affairs."The federal government welcomes the prevailing keen interest and partnerships for successful elections in, and a peaceful Nigeria," said presidential spokesman Garba Shehu."However, we reject any interference or perception management that promotes apprehension,citizens distrust or undermines the transparency and acceptability of the outcomes of our electoral process," he said."Nigeria reserves the right to be insulated from suggestions and or interference with respect to wholly internal affairs." 'Attempted coup' Buhari, of the All Progressives Congress (APC), came to power in 2015 on a pledge to stamp out corruption and is seeking another four-year term.He was out campaigning in southwest Nigeria on Saturday. His main challenger, Atiku Abubakar, 72, from the Peoples Democratic Party (PDP), however, suspended his campaign for 72 hours in protest at Onnoghen's removal.On Friday, he called the move "a brazen act of dictatorship" and "anti-democratic", in a clear reference to Buhari's army past and autocratic rule in the 1980s.Using similar language, the Nigerian Bar Association - the umbrella body for lawyers - said Buhari had mounted an "attempted coup against the Nigerian judiciary".The independent Punch newspaper said the move could trigger "an unnecessary constitutional crisis and, perhaps, derail 20 unbroken years of democratic governance".Nigeria returned to civilian rule in 1999 after decades of military government.Buhari has previously been accused of using state security apparatus to target political opponents from the PDP, but being less swift to act against members of his own party.Buhari said he suspended Onnoghen after receiving an order from the Code of Conduct Tribunal hearing the case, despite a Court of Appeal injunction ordering the lower court to suspend proceedings while an application to throw out the charges was heard.The NBA warned the executive arm of government's actions were unconstitutional and "portends a slide into anarchy".The Situation Room group of Nigerian civil society organisations called for the emergency recall of parliament to give a "legislative response".Any return of lawmakers would likely set up another confrontation between the legislature and executive, as Senate leader Bukola Saraki is a prominent Buhari opponent.He was cleared of similar non-declaration of assets charges on appeal at Onnoghen's Supreme Court last year.Moral authority Under the Nigerian constitution, the Senate has to approve an application for the removal of the chief justice by a two-thirds majority.Law professor Itse Sagay, who heads a government-appointed advisory body on graft, defended Buhari, who indicated Onnoghen should have recused himself after being charged.The judge had admitted a failure to comply with assets declaration requirements, he said, adding: "The suspension is morally justified and legally justified."Debo Adeniran of the Coalition Against Corrupt Leaders lobby group also backed the president, saying the suspension would "send a correct signal to other judges to sit up", he said.Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook.