Moscow Deputy Mayor Alexander Gorbenko warned organizers of Saturday's rally against election results that the number of protestors should not surpass the previously declared 300 people.
Some 20,000 people have so far indicated on social network sites that they will attend a sanctioned protest on Saturday at central Moscow's Revolution Square.
"A total of 300 participants were declared to take part. If more people are eager to join, organizers will be held responsible. We will warn people that they attend an unsanctioned part of the rally, and are thus subject to all measures envisaged by the law," the deputy mayor said in an interview with the Ekho Moskvy radio station.
Gorbenko invited organizers to apply for another rally location if they expect more people to come.
"We will find an opportunity to choose a more suitable place together," he said, adding that it was only a matter of public security.
The deputy mayor said the organizers face a fine or an administrative arrest of up to 15 days for violating the law on public protests.
Demonstrations against alleged electoral fraud in favor of Prime Minister Vladimir Putin's United Russia party continued for a third night on Wednesday, with arrests in both Moscow and St. Petersburg, police said.
The deputy mayor also dismissed media reports that the square would be closed for "repairs" before Saturday. According to a RIA Novosti correspondent, who visited the site of the future protest, an area of 50 by 70 meters is currently closed.