Mikhail Gorbachev called for a re-run of Russia's parliamentary election on Wednesday, accusing officials of "falsification and rigging".
Up to 15,000 demonstrators prepared to take to Moscow's streets this weekend for another round of protests over the results.
With popular anger over the elections showing no signs of abating, the last leader of the Soviet Union added his voice to those from the country's liberal opposition who have demanded the annulment of an election that international observers said was riddled with fraud.
As Russian-language social networking sites indicated last night that anti-Kremlin activists were trying to organise at least 81 protests in other Russian towns and cities this Saturday, Mr Gorbachev said: "The results do not reflect the will of the people.
"Therefore I think they (Russia's leaders) can only take one decision – annul the results of the election and hold new ones. With every day that passes more and more Russians do not believe that the official results were honest.
"Ignoring public opinion discredits the authorities and is destabilising the situation."
The elections have sparked unusually large opposition protests in Moscow, taking the authorities by surprise. The poll saw support for the ruling United Russia party, aligned to Vladimir Putin, the prime minister, haemorrhage by 15 percentage points. The protesters, who numbered up to 10,000 people on Monday, have demanded new elections and Mr Putin's resignation.
Germany, the Kremlin's closest ally in Europe, condemned the vote yesterday, urging Moscow to put its house in order. Steffen Seibert, a spokesman for Chancellor Angela Merkel, echoed American criticism of the poll, saying the election was marred by "frequent procedural violations."
Germany "expects that Russia will live up to its democratic obligations as a constitutional state," he added. "That includes equal opportunities for political opponents, the right to protest and the right to a fair trial."
But the Kremlin has shown few signs of yielding. On the contrary, Mr Putin offered an ostentatious show of defiance, visiting the central election committee to submit his application to run for president in March, an election he is expected easily to win.
Polls show he remains the country's most popular politician; he has not even deigned to pass comment on three days of opposition protests in the capital. On the contrary, state TV, which most Russians still rely on for their news, has studiously ignored the demonstrations, while providing its usual fawning coverage of Mr Putin and rallies of his supporters.
Around 1,000 people have been arrested in the last few days and, in some cases, sentenced to jail terms, but many Russian voters will know nothing about this.
"I cannot remember a more total news blackout in recent times," said Arina Borodina, a television reviewer, on the Kommersant FM radio station.
Its political correspondent, Stanislav Kucher, furiously denounced journalists at state channels for their selective reporting, urging them to return the awards they had picked up in recent years,
"You are hiding information from millions of people," he said. "You are disgracing yourselves and your profession."
A small number of opposition protesters was again swept off the streets by heavy-handed riot police last night. Eye-witnesses said that no more than 50 people turned up - and 20 arrests were made.
Russia's main news bulletin chose to ignore these disturbances and open with an item of old news – the fact that Moscow is expanding its city limits. A more liberal TV station, Rain TV, looked as if it was already paying a price for its more open approach, saying that regulators had already demanded its tapes of coverage of the recent anti-Putin protests.
Meanwhile, more than 50,000 police and special interior ministry troops remained on Moscow's streets, sealing off the square where the protests have taken place. Police helicopters buzzed over the capital all day, monitoring the movement of opposition supporters, while the city's aptly-named Revolution Square, where Saturday's big protest is due to unfold, was abruptly sealed off for repair work. Officials denied it was a deliberate attempt to sabotage the rally, promising it would be re-opened soon.
Russia's foreign ministry shot back at its critics, calling criticism of the election "politicised" and "questionable." Sergei Jeleznyak, a senior MP from Mr Putin's party, alleged that the protesters were being encouraged by hostile foreign powers, insisting that the number of irregularities during the election had been insignificant. "Professional provocateurs financed by the enemies of our country are actively stirring things up.
Our enemies oppose the peaceful development of our society and a strong independent Russia," he said.