Some 2,000 people protested in central Moscow on Sunday as opposition candidates accused the city authorities of seeking to remove them from the ballot in elections for the city legislature.
The unauthorised protest, at the central Pushkin Square, was organised by a group of candidates including supporters of opposition leader Alexei Navalny.
While police turned out in force, they opted not to break up the rally. Candidates have the right to organise meetings with the electorate without sound amplification or banners.
Russia is set to hold nationwide elections for local and regional legislatures in September.
Protesters say the electoral authorities have falsely accused them of faking signatures of members of the public required for independent candidates to stand in September polls.
In Russia, candidates outside the large parliamentary parties have to gather large numbers of signatures from supporters to be allowed to stand. The details of signatories are then checked for authenticity by the electoral commission.
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"Across the whole of Moscow now, they are removing independent candidates," said opposition politician Ilya Yashin, currently a local councillor and one of the candidates who backs Navalny.
"Starting from tomorrow, the electoral commission will start removing us all from the election," Yashin said.
Protesters took it in turns to knock on the heavy wooden door of the mayor's office on the city's main Tverskaya street to demand candidates be allowed to stand. No one opened up.
Around a thousand protesters then rallied outside the city electoral commission.
One of the candidates, Lyubov Sobol, an ally of Navalny, on Saturday declared a hunger strike after she was told the number of allegedly fabricated signatures backing her candidacy exceeded the permitted proportion.
Moscow city electoral commission said in a statement Sunday that more than 700 of 4,940 signatures backing Sobol were "invalid".
"They are stealing these elections, they are stealing our future," Sobol warned.
"I won't retreat... I will stand and fight until the end" she told a cheering crowd. One candidate was detained by police en route to the rally, said OVD-Info, a site that monitors detentions at protests via a hotline.
Elections for Moscow city parliament are being contested by high-profile opposition politicians who hope to capitalise on a mood of public discontent and falling approval ratings for President Vladimir Putin.