The last Soviet leader Mikhail Gorbachev has called on Russia Prime Minister Vladimir Putin to heed protester demands and quit politics instead of seeking a third term as president next year.
"I would advise Vladimir Putin to leave now. He has had three terms: two as president and one as prime minister. Three terms -- that is enough," Mr Gorbachev told Moscow Echo radio.
His comments come after thousands of people took to the streets of Moscow to call for disputed parliamentary elections in the country to be rerun.
Police estimated that more than 20,000 people gathered in the Russian capital, however organisers say the number was closer to 120,000.
The demonstrations increase pressure on Mr Putin as he seeks a new term as Russian president.
The protesters have been heartened by the Kremlin's human rights council saying a new election should be held - although many realise it is an advisory body whose recommendations are regularly ignored by Russia's leaders.
Tens of thousands of people also protested in Moscow earlier this month to complain against alleged vote-rigging in the election won by Putin's United Russia party.
Protesters gathered this morning on Prospekt Sakharova (Sakharov Avenue) - named after Soviet-era dissident Andrei Sakharov - where a large stage was erected with a slogan declaring: "Russia will be free!".
Around 50,000 attended the last rally in Moscow on 10 December.
Bringing together liberals, nationalists, anarchists, environmentalists and urban youth, protest organisers have settled on a list of 19 speakers that includes former Soviet President Mikhail Gorbachev, anti-corruption blogger Alexei Navalny, rock singer Yuri Shevchuk and writer Boris Akunin.
The opposition has rejected conciliatory efforts by Mr Putin and President Dmitry Medvedev - saying they have ignored its key demand for a rerun of the parliamentary poll, which handed a slim majority to the ruling United Russia party.
The opposition says United Russia benefited from widespread voting irregularities and international monitors said the vote was slanted in the ruling party's favour.
The Kremlin human rights council said the alleged irregularities discredited the new State Duma lower house.
It said this posed "a real threat to the Russian state" and called for the resignation of Vladimir Churov, the head of the central election commission.
