Russians furious about Vladimir Putin's return to the presidency fear a wider crackdown after hundreds were arrested following a post-election protest.
"It is absolutely certain that the period of peaceful protests and marches is over," the novelist Boris Akunin, a member of the protest organising committee, told Kommersant FM radio.
He criticised riot police, deployed by the thousand to the capital during and after the vote, for violently detaining hundreds of protesters on Monday. Future protests would lead to "a clear manifestation of aggression from the authorities", he said, a testament to their "nervousness".
Putin's spokesman defended the arrests, saying the helmet-clad riot police had acted with a "high level of professionalism, legitimacy and effectiveness".
The Russian foreign ministry took it further, writing directly to the US ambassador to Russia, Michael McFaul, to criticise the New York police department's handling of Occupy Wall Street protesters. "The police on Pushkin Square were many times more humane than what we saw during the breakup of Occupy Wall Street, tent camps in Europe," the ministry wrote.
Opposition activists said they feared authorities would use means both legal and extralegal to squash the anti-Putin movement that exploded after a disputed parliamentary vote in December.
The prosecutor general's office said it would investigate whether speakers at the rally on Monday had provoked "mass unrest". Riot police detained hundreds of people who had refused to leave Moscow's Pushkin Square following an anti-Putin protest that gathered around 20,000 people.
Among those detained were the opposition leaders Alexey Navalny and Sergei Udaltsov. They were released hours later. Navalny's court date has been set for 15 March.
"Investigators are currently studying videotapes of the events that happened on Pushkin Square, questioning participants in the protest and police officers that were securing the public order," prosecutors said in a statement. If charges are brought, the guilty could face up to two years in jail.
Police have already swooped on other activists. Six members of the feminist punk band Pussy Riot were arrested at the weekend, suspected by police of "gross violation of the public order and religious hatred" following an unsanctioned performance in Moscow's Cathedral of Christ the Saviour. Two remain in custody, and have launched a hunger strike. They could face up to seven years in jail if police bring charges of hooliganism.
Protesters also suspected that the campaign against them had been taken to the internet. Dr Web, a Russian IT security firm, said it had flagged a new computer virus sent in emails calling on people to attend the protests.
Also on Tuesday, Andrew Ryvkin, a political blogger for the Russian edition of GQ, said he was attacked by two men known for their pro-Kremlin views. The writer Sergei Minayev and Eduard Bagirov, a member of Putin's election campaign, admitted to the attack on Twitter, but said that it involved "no politics".
Despite the pressure, protesters have vowed to continue their calls for new elections. A fresh protest has been set for 10 March, but organisers have yet to receive approval from city authorities. After the attempt to occupy Pushkin Square, organisers indicated that they may carry out unsanctioned protests in future.
"It is necessary to start a movement to return to us the city that we live in," they wrote on their Facebook page. "There is currently no authority in Moscow that represents the interests of Muscovites."
Putin has refused to address the protesters' concerns, saying on Tuesday that the fraud claims were "an element of political struggle. It has no relation to the election."
"Putin doesn't really believe that something changed," said Andrei Soldatov, an analyst specialising in the Russian security services. "He clearly doesn't understand what happened in September."
Although many Russians expected Putin to seek a return to the Kremlin, popular anger grew with his announcement in September that he would run for re-election. "Putin will try to return the situation to as it was before, in the summer of 2011," Soldatov said. "But I don't think that's possible." He said sources inside the security services were becoming increasingly nervous that the opposition would seek to capitalise on a fresh expression of discontent during Putin's planned inauguration in early May.
Akunin, the novelist, said the authorities' nervousness was palpable, prompting the show of force on Monday. "Putin's victory in the first round is becoming more and more doubtful," he said. Activists and election monitors continued to pore over reports of falsifications, revealing rates in the troubled Caucasus republics that rarely dipped below 97% support for Putin. "I think exactly this explains the authorities' nervousness," he said.
