Tens of thousands of protesters have gathered in the Russian capital to challenge Prime Minister Vladimir Putin's grip on power, a month before he stands in presidential elections.
Activists braved freezing temperatures on Saturday as they began marching though Moscow in a bid to demand a free and fair presidential poll.
Government supporters, meanwhile, began to gather in the west of the city for a rival rally, amid increasing complaints claiming that teachers, nurses and state employees had been offered cash incentives or even given orders to attend the pro-Putin event.
"Between 87,000 and 90,000 are attending the [pro-Putin] rally that has started at Poklonnaya Gora" in western Moscow, capital city police said in a statement. "Around 23,000 are taking part in the [opposition] march."
Elsewhere, hundreds of Russian opposition members filled the streets of the far eastern city of Vladivostok, calling for fair elections.
The third mass anti-Putin rally in less than two months will be seen as a crucial test of the nascent protest movement's ability to keep up its momentum despite the prime minister's refusal to bow to opposition demands that include a re-run of December parliamentary polls.
The two previous opposition rallies in Moscow on December 10 and 24 gathered tens of thousands of people in the largest protests of Putin's 12-year rule as president and prime minister.
Harsh cold spell
The new demonstration comes after the country's 10-day New Year holidays and in a harsh cold spell, with temperatures in Moscow expected to hover around -17C.
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 the AFP news agency.
"It is also because it's very cold in Moscow and this march will show just how freeze-proof our sense of dignity and freedom is."
For many Russians, Saturday's rally carries special symbolism because it coincides with 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.
Cap on demonstrators
Veteran liberal politician Grigory Yavlinsky, who has been excluded from the March 4 presidential election by the central election commission on procedural grounds, is expected to speak at Saturday's protest.
Billionaire Mikhail Prokhorov, the only independent allowed by election officials to run, also said he would be there.
More than 27,000 people said on Facebook that they would attend the march, which will move from the southern end of the city centre to a square just over the Moscow river from the Kremlin, where the event will close with a rally.
Authorities have said no more than 50,000 can take part in the anti-Putin demonstration and have made a series of calls to dissuade Russians from turning up.
Putin's supporters are hoping tens of thousands show up for the rival rally in Poklonnaya Gora War Memorial Park in a demonstration called the "anti-Orange protest", in a reference to Ukraine's 2004 Orange Revolution that ousted its old order from power and infuriated the Kremlin.
Complaints have multiplied in recent days that employees of state companies are being offered cash incentives or have been ordered to attend the pro-Putin rally. Authorities have denied the accusations.