Former Soviet President Mikhail Gorbachev has said Sunday's Russian parliamentary election was marred by fraud and has called for a re-run.
"The country's leaders must admit there were numerous falsifications and rigging and the results do not reflect the people's will," he said.
Protesters were planning new rallies for Saturday as arrests in Tuesday's crackdown in reached 800 across Russia.
Key figures in the protest movement are starting 15-day jail sentences.
State TV channels have ignored the protests, giving coverage only to rallies in support of the government.
The centre of Moscow saw its biggest protest against the rule of Prime Minister Vladimir Putin and his allies in years on Monday, when several thousand people came out to condemn widely reported fraud at Sunday's parliamentary elections.
An attempt to hold a smaller rally on Tuesday was quickly broken up by riot police, while rival rallies by Putin supporters were allowed to proceed.
The big question now is whether the fledgling protest movement can maintain its momentum.
While the movement began as a protest against the election results, most of the slogans have been against Mr Putin, our correspondent adds.
The Russian prime minister, who formally registered on Wednesday to stand in the March presidential election, has played down losses by his party, United Russia, which saw its support drop sharply to just under 50% of the vote.
OSCE monitors have said the polls were slanted in favour of United Russia, noting apparent manipulations such as the stuffing of ballot boxes.
Mr Gorbachev told Russian news agency Interfax in Moscow: "I think they [Russia's leaders] can only take one decision - annul the results of the election and hold a new one."
The former leader, 80, initiated democratic reforms in the final years of the USSR but rapidly lost popularity and influence after the emergence of the new Russian state under the late Boris Yeltsin.
"Literally by the day, the number of Russians who do not believe that the declared election results were honest is increasing," he said.
"In my opinion, disregard for public opinion is discrediting the authorities and destabilising the situation."
Messages on Twitter and other social media, used to co-ordinate the earlier protests, are fixing Saturday as the date for the next demonstrations.
Revolution Square, just 200m from the Kremlin, was named as the venue in Moscow.
Unconfirmed reports in the Russian media on Wednesday said the city authorities were planning to shut down the square's metro station of the same name "for repairs".
According to the Russian news website lenta.ru, the Solidarity opposition party has received permission from the city authorities to hold a rally on the square limited to 300 people.
However, the party's Facebook page "Rally For Honest Elections" had received nearly 16,000 requests to attend the rally as of Wednesday afternoon.