Two exit polls have suggested that Kremlin ally Sergei Sobyanin has won the Moscow mayoral election.
However, opposition leader Alexei Navalny did much better than expected and has challenged the count.
The exit polls put Mr Navalny on about 30% of votes, a strong showing that could energise resistance to President Vladimir Putin.
Mr Navalny's campaign team said it believed Mr Sobyanin had not crossed the 50% threshold for outright victory.
An exit poll by state-funded VTsIOM showed Mr Sobyanin won 53% of votes against 32% for Mr Navalny.
Itar-Tass said an exit poll by the Public Opinion Foundation gave Mr Sobyanin 52.5% and Mr Navalny 29.1%.
But Mr Navalny's aides said their own exit polls showed Mr Sobyanin received only 46%.
Mr Navalny, 37, faces a five-year jail term if he loses an appeal against a July conviction of theft from a state firm.
He denies the charge and said his trial was Kremlin revenge for his anti-corruption efforts and opposition activity.