Anger against Vladimir Putin's ruling party and alleged election fraud have boiled over into a third straight night of protests, and police in Russia's two largest cities have arrested scores of demonstrators.
The demonstrations in Moscow and St. Petersburg on Wednesday appeared to attract fewer protesters than in previous days - roughly 300 in each city - but Russians' willingness to risk jail time and clashes with police indicated significant tensions that could spread.
More than 17,000 people have signed up for a Facebook page calling for a massive demonstration on Saturday on Moscow's Revolution Square.
Authorities have sanctioned the rally, but say it has to be limited to 300 participants, so a far larger turnout would almost certainly provoke a harsh police response.
Mr Putin's United Russia party lost a significant share of its seats in Sunday's parliamentary election for the State Duma, the lower house of parliament, but will still have a majority. Opponents say even that result was achieved by widespread vote fraud.
United Russia had two-thirds of the seats in the 2007 vote, making the party unassailable and allowing it to push through constitutional changes.
The latest protests came hours after former Soviet leader Mikhail Gorbachev urged Russian authorities to annul the results of Sunday's vote and hold a new one, according to the Interfax news agency.
"More and more people are starting to believe that the election results are not fair," he told Interfax. "I believe that ignoring public opinion discredits the authorities and destabilizes the situation."
Mr Gorbachev, whose 1985-91 rule ended with the collapse of the Soviet Union, remains widely admired abroad but is widely disliked or regarded as insignificant at home. His call for a new election could further encourage the opposition, but is unlikely to influence those in power.
The 80-year-old Mr Gorbachev has long had tense relations with Mr Putin, but until recent years had refrained from directing his criticism of Russian politics at him. Mr Putin, for his part, has been extremely critical of Mr Gorbachev's legacy, blaming him for the Soviet Union's demise.