Around 8,000 people protested in Moscow and Saint Petersburg on Sunday against what they say were rigged parliamentary polls that handed victory to Vladimir Putin's ruling party.

An opposition activist, Sergei Udaltsov of the Left Front, was meanwhile in critical condition after he went on hunger strike since being detained early this month, his lawyer and wife said.

The new rallies came on the heels of a wave of protests that swept Russia the previous weekend after the opposition and independent observers said Putin's United Russia party had cheated its way to a slim majority in December 4 parliamentary polls.

More than 3,000 people attended a rally on Manezhnaya Square near the Kremlin walls organised by the Communist party, the runner-up in the parliamentary elections.

Yury Molodkin said he joined the rally because he was "outraged" by Putin's claims that protesters were in the pay of a foreign state and compared a symbol of the protests against his rule -- the white ribbon -- to condoms.

"I came to listen to people who are ready to fight these liars," the 46-year-old told.

In a live televised phone-in beamed across Russia on Thursday, Putin claimed he wasn't troubled by the largest protests of his 12-year rule.

"He talked like a crime boss," said Molodkin, who noted he had not been to a rally since 1993.

Police put the turnout at the Moscow protest at 3,300 people.

In Russia's second city of Saint Petersburg protesters chanting "Russia Will Be Free!" and holding signs such as one reading "Give Back My Vote!" also said they were offended by Putin's claims they were hired to protest.

"It's a ridiculous idea that people get paid for coming here," said Sergei, a student and one of some 5,000 protesters who gathered in the central Pionerskaya square.

"Clearly everyone is tired of the lies coming from the authorities," said Sergei, who held a sign reading "I Was Asked to Stand Here for $10".

More than 50,000 people gathered in Moscow the previous weekend in the biggest show of popular anger since the turbulent 1990s. The next major opposition protest in Moscow is scheduled for Saturday.

The leader of one of the opposition groups that organised the large Moscow rallies was earlier in the day hospitalised and fighting for his life in an intensive care unit, his wife and fellow activist Anastasia Udaltsova and his lawyer said.

"Udaltsov was examined for several hours," his lawyer Violetta Volkova said on Echo of Moscow radio. "Doctors decided to put him into an intensive care unit."

Udaltsov has been on hunger strike since being detained on December 4 for participating in an unsanctioned rally.

Both Putin and President Dmitry Medvedev have rejected the protesters' claims of mass violations during the vote, with Putin also accusing US Secretary of State Hillary Clinton of sparking the rallies by questioning the vote's legitimacy.

The newly elected lower house of parliament, the State Duma, is scheduled to convene for its first session on Wednesday.

The protests come less than three months before March presidential polls in which Putin is widely expected to reclaim his old Kremlin job.

Amid the worst legitimacy crisis of his rule, Putin's approval ratings have however taken such a dive that, according to most recent opinion polls, he will not be able to secure victory in the first round. (AFP)