Tens of thousands say they are prepared to take to the streets in the biggest challenge yet to the country's government

Russia's anti-government protest movement has gathered momentum as tens of thousands of people said they were prepared to take to the streets this weekend in the biggest challenge to Vladimir Putin's rule.

With concern inside the Kremlin growing, Putin and Dmitry Medvedev, Russia's president, met their security council, including the interior and defence ministers, the head of the federal security service (FSB) and the country's foreign intelligence chief, to discuss the situation.

Helicopters hovered in the skies over Moscow, while the police presence on the streets of the Russian capital remained strong following two protests that led to hundreds of people arrested.

The movement was triggered by a disputed parliamentary election result that protesters say wildly overstated the popularity of Putin's United Russia party.

Mikhail Gorbachev, the former premier who oversaw the end of the Soviet Union, on Wednesday called on the Russian authorities to annul the election result and hold a new vote. "More and more people are starting to believe that the election results are not fair," he told the Interfax news agency. "I believe that ignoring public opinion discredits the authorities and destabilises the situation."

The authorities "must admit that there have been numerous falsifications and ballot stuffing", he added.

More than 16,700 people indicated on Facebook their intention to gather in Revolution Square, a stone's throw from the Kremlin, this Saturday. Another 5,500 said they would attend a similar protest in St Petersburg. Protests were also being organised in more than 80 cities across Russia, including the Urals city of Yekaterinburg and the Siberian city of Surgut.

But there was growing fear that the Russian authorities would step up their action against protesters.

Yevgenia Chirikova, an environmental activist turned opposition leader, said she feared the Kremlin would move beyond the haphazard arrests and the deployment of pro-Kremlin youth groups that have so far marked its response to the protests. She noted that the government had recently approved a pay increase for the army, a move designed to ensure its loyalty.

"Putin has no other choice than to hold on to power, shoot himself or sit in jail," she said. "The system he has built is so corrupted, and there have been so many crimes, that there is no other path. He will fight for his power."

Two of the opposition's leaders, Alexei Navalny and Ilya Yashin, remain jailed.

Vladimir Milov, another opposition leader, warned people to stay away from Saturday's protest.

"All this can end in big blood," he wrote on the website of Ekho Moskvy, a liberal radio station. "This is the most dangerous thing in today's situation."

He urged those protesting over the election result to direct their anger at the 4 March presidential vote, which is expected to sweep Putin back into the Kremlin. "Blood and unrest can throw the 'Russian spring' far back," Milov wrote. "We don't need that now."

Anger has been galvanised in the wake of a parliamentary election on Sunday which saw United Russia slip below 50% – despite reports of fraud which indicate its support has fallen much further.

Opposition activists appeared to be taken aback by the swell of popular support. "It's hard to predict what will happen further," Chirikova said. "This is the first time we've seen such events after an election. That people are grabbing on to these falsifications is a new trend in Russian society."

Dmitry Finikov, a 31-year-old small-business owner who volunteered as an election monitor, uploaded his tale to LiveJournal on Tuesday morning and it was viewed more than 150,000 times by Wednesday.

His photographs and video show how election officials at his polling site in central Moscow threw away the official result of the vote, which showed United Russia coming in third with 128 votes, behind the Communists (202) and liberal Yabloko (134). In the final tally, United Russia won the vote at the site.

Yabloko failed to pass the threshold to enter the Duma (lower house of parliament), and members of the party have joined the protest movement.

"I wrote everything I saw, I just did what I had to do," said Finikov, who will attend his first ever anti-government protest on Saturday. "No one in Russia thinks elections are honest. But it shouldn't be this way. It must not be this way."

Officials continued to reject the charges of fraud. Vladimir Churov, head of the elections commission, said in an interview published on Wednesday that the videos of fraud were themselves fraudulent.

"There is a lot of rubbish on the internet about violations now," he told Itogi magazine. "Even before the vote, I knew of several fake 'polling sites' in flats, where they shot these movies. I think we'll see more of them."

On Wednesday Putin filed papers declaring his candidacy for the presidential vote, failing to deliver any of the blustery comments for which he has become known. Putin's widely expected return to the presidency has hit a nerve with many of Russia's urban, well-educated youth, who are dismayed at the prospect of him potentially running the country for another 12 years.