The Kremlin said a call by opposition leader Alexei Navalny to boycott next year's presidential election must be checked to see if it complies with the law, paving the way for possible police action against him and his supporters.
"The calls for a boycott will require scrupulous study, to see whether or not they comply with the law," Kremlin spokesman Dmitry Peskov told reporters on a conference call.
Declining to comment on the election commission's decision, Peskov shrugged off allegations that the presidential poll would be a farce without Navalny.
"The fact that one of the would-be candidates is not taking part has no bearing on the election's legitimacy," said Peskov.
On Monday Navalny said millions of voters would be disenfranchised without his participation in the election, which opinion polls show incumbent Vladimir Putin winning comfortably.
The European Union also questioned the Russian election commission's decision to bar Navalny.
"(It) casts a serious doubt on political pluralism in Russia and the prospect of democratic elections next year," the EU's External Action Service said in a statement on Tuesday.
"Politically motivated charges should not be used against political participation," it said, urging Moscow to ensure a "level playing field" for all Russian elections.
Alexei Navalny was ruled ineligible to take part due to a suspended prison sentence he maintains was politically motivated.
The decision by the central election commission on Monday was widely expected as election officials had repeatedly declared Mr Navalny would be ineligible to run.
Twelve members of the 13-member commission voted to bar Mr Navalny. One member abstained, citing a possible conflict of interest.
Mr Navalny, 41, who polls show would struggle to beat incumbent Vladimir Putin in the March election, said he would appeal and called on his supporters to boycott the election and campaign against it being held.
"We knew this could happen, and so we have a straight-forward, clear plan," Mr Navalny said in a pre-recorded video released immediately after the decision.
"We announce a boycott of the election. The process in which we are called to participate is not a real election. It will feature only Putin and the candidates which he has personally selected."
Mr Navalny said he would use his campaign headquarters across Russia to support the boycott and monitor turnout on voting day, March 18.
Polls show Mr Putin, 65, who has dominated Russia's political landscape for the last 17 years, is on course to be comfortably re-elected, making him eligible to serve another six years until 2024, when he turns 72.
Allies laud Mr Putin as a father-of-the-nation figure who has restored national pride and expanded Moscow's global clout with interventions in Syria and Ukraine.
Mr Navalny says Mr Putin's support is exaggerated and artificially maintained by a biased state media and an unfair system. He says he could defeat him in a fair election, an assertion Putin's supporters have said is laughable.
There had been some speculation prior to the decision among the opposition Mr Navalny might be allowed to run in order to inject more interest into what looks like a predictable contest amid Kremlin fears that apathetic voters might not bother to vote.
Mr Navalny has been jailed three times this year and charged with breaking the law by repeatedly organising public meetings and rallies.