Fears were growing of clashes between police and demonstrators over today's presidential election in Russia, after calls to escalate protests against the government of Vladimir Putin.
The decision of senior co-ordinators to agree to a sanctioned protest rally in Moscow's Pushkin Square tomorrow, rather than under the walls of the Kremlin, has angered opposition supporters.
A core of activists plans to march to the seat of the presidency and set up tents in protest at Mr Putin's rule and the rigging, which election watchdogs predict will take place to secure him a win in the first round of the vote.
Vadim Korovin, 34, an opposition activist, said: "We need to rise up and change our tactics. We need to put up tents."
Mr Korovin said: "Commissars from Nashi [a fanatical Pro-Putin youth group] are already sharpening their fists. They told me they will stop people pitching tents and carry them off hand and foot."
Disorder could potentially spread because of the highly charged atmosphere anticipated as rival demonstrations are held tomorrow.
Police have drafted 6,500 reserves, including special forces from Russia's regions, and up to 35,000 officers will be on duty.
Bloggers posted photographs of columns of police trucks trundling toward the capital yesterday. Ilya Ponomarev, an opposition leader and MP, said security sources informed him that 800 Chechen troops had been brought to the capital as a reserve to help crush protests if necessary. Nashi has said it will put several thousand members on Moscow's streets to prevent "illegal attempts by the opposition to destabilise the situation".
Mr Putin's circle has cleared the ground for a harsh response to any dissent resembling the 2004 Orange Revolution in Ukraine when demonstrators camped near Kiev's Independence Square.
A senior Kremlin source told Kommersant newspaper: "If there are any attempts to move to longer-lasting protests like the Ukrainian one, tough measures will be taken."
After long talks, protest co-ordinators agreed with Moscow authorities that tomorrow's opposition protest would be held on Pushkin Square at 7pm, rather than Manezh Square next to the Kremlin which they had originally requested.
That decision provoked a sharp reaction from part of the protest movement. Speaking on condition of anonymity, one anti-Putin activist said: "There was no need to compromise."
Mr Putin claimed last week that protesters were planning "provocations".
"They'll stuff the ballots," he said, "and then complain about it."
However, election monitors expect the ruling authorities to be the chief culprit in any fraud and tens of thousands of civil observers will scrutinise the vote.