Tens of thousands of Russians were to march in Moscow Saturday, braving freezing temperatures to challenge Vladimir Putin's grip on power a month before the premier stands in presidential elections.
Their third mass rally in less than two months is seen as a crucial test of the movement's ability to keep up momentum despite Putin's refusal to bow to its demands that include a re-run of December parliamentary polls.
Government supporters will stage a rival rally dubbed the "anti-Orange protest" -- a reference to Ukraine's 2004 Orange Revolution that ousted its old order from power and infuriated the Kremlin.
Putin thanked his supporters on the eve of the protest and urged them to dress warmly.
"I am grateful to them and share their views," he said late Friday.
Complaints have multiplied in recent days that employees of state companies are being offered cash incentives or even being ordered to attend the pro-Putin rally.
Opposition newspaper Novaya Gazeta reported that even parliament members and their aides were forced to join the pro-Putin rally in Poklonnaya Gora War Memorial Park despite freezing temperatures.
The newspaper quoted an aide to a senator, who spoke anonymously, as saying "those above have lost their (expletive) mind".
Putin admitted Russians may at times be pressured to join pro-government rallies but said reports of pressure should not be exaggerated.
By contrast, an opposition rally is being called through social networks, with more than 27,800 saying on Facebook they would attend.
The two previous opposition rallies in Moscow on December 10 and 24 gathered tens of thousands in the largest protests of Putin's 12-year rule as first president and then prime minister.
The new demonstration comes after the country's 10-day New Year holidays and amid a harsh cold spell, with temperatures in Moscow expected to hover around minus 17 degrees Celsius (1.5 Fahrenheit).
The organisers are hoping the big freeze will not discourage Russians from taking part in the march and subsequent rally.
"It will be a very important event," prominent detective novelist and opposition activist Boris Akunin told AFP. "This march will show just how freeze-proof our sense of dignity and freedom is."
Police said thermos flasks of hot tea and coffee will not be allowed at the demonstration for safety reasons.
For many Russians, Saturday's rally carries special symbolism because it falls on the anniversary of a February 4, 1990 rally when up to 300,000 took to the streets just before the fall of the Soviet Union to demand the end of the Communist Party's grip on power.
Authorities have said no more than 50,000 can take part in the anti-Putin demonstration and have made calls to dissuade Russians from turning up.
"Today all of us are determining Russia's future," Boris Gryzlov, a top official in the ruling United Russia party, said Friday.
"I would like to tell my compatriots -- if the future of Russia and that of our children is dear to you, keep away from all those swamps," he said in a reference to the venue of the rally, Bolotnaya Ploshchad (Swamp Square).
Russia's Orthodox Church, which has been watching the opposition calls to demonstrate with growing unease, said Orthodox Christians do not take to the streets but "pray in the quiet of monasteries, cells, and homes."
Smaller anti-Putin and pro-Putin rallies are planned outside Moscow, with the biggest turnouts expected in Saint Petersburg, the Urals city of Yekaterinburg and the biggest Siberian city Novosibirsk.
Some 200 Russian citizens and members of political parties marched in the Pacific port of Vladivostok, carrying signs with slogans including "Putin will be fired" and "Putin resign."