Thousands of people are marching in Russia's capital Moscow in protest at Prime Minister Vladimir Putin's grip on power.
It is the third rally since December's parliamentary election was marred by allegations of vote-rigging.
But supporters of Mr Putin, who will stand in next month's presidential election, are holding their own rally at a different location.
People at the rallies will be braving temperatures as low as -19C.
Both the organisers of the "For Honest Elections" march and Mr Putin are hoping the freezing temperatures do not affect the turn-out, but urged their respective supporters to wrap up warmly.
"The main thing is for people not to catch pneumonia... Three hours in the cold is a serious thing," said liberal politician Boris Nemtsov, an organiser of the opposition rally.
Moscow's Kaluzhskaya Square was filling up with protesters carrying white balloons - the colour adopted by the protest movement - for the start of the march.
The turnout is seen as a key indicator of whether the protest movement against Mr Putin still has momentum, observers say.
As many as 50,000 people turned out in Moscow for the first rally, following 4 December elections, which were won by Mr Putin's party.
Despite government denials of widespread vote-rigging, protests spread to other cities.
The organisers of Saturday's protest are demanding a re-run of December's election, and calling on people to vote against Mr Putin in March's presidential election.
The BBC's Steve Rosenberg in Moscow says the organisers do not expect to be able to stop Mr Putin from winning March's election, but they hope they can pile the pressure on him to institute political reform.