Moscow is braced for what the opposition claims will be the biggest demonstration in Russia for 20 years.Tens of thousands are gathering in a square south of the Kremlin, in the latest show of anger over disputed parliamentary polls. Smaller rallies are taking place in cities across the country.
Protesters allege there was widespread fraud in Sunday's polls - though the ruling United Russia party saw its share of the vote fall sharply.
Hundreds of people have been arrested during anti-Putin protests over the past week, mainly in Moscow and St Petersburg.
At least 50,000 police and riot troops have been deployed in Moscow ahead of Saturday's protests.Authorities have permitted up to 30,000 to attend the demonstration dubbed "For Fair Elections".
Earlier, several hundred people marched in Vladivostok, seven timezones to the east of Moscow.
The BBC's Daniel Sandford in Moscow says in the past week, the city has resembled a police state rather than a democracy.If the protests come even close to expectations, they will shake the 12-year-long political domination of Prime Minister Vladimir Putin, he says.
The authorities permitted demonstrations to take place in specific locations in certain cities after negotiations with opposition leaders.
In Moscow, the two sides reached a deal by which authorities would allow a high turnout if the rally was relocated from downtown Revolution Square to Bolotnaya Square, a narrow island in the Moscow River where access points can be easily controlled.
Lots of demonstrators - among them communists, nationalists and liberals - are gathering in Revolution Square and walking to Bolotnaya Square, reports the BBC's Steve Rosenberg. Hundreds of police are standing by to make sure they don't rally in Revolution Square.
In St Petersburg, 13,000 people have pledged on the social networking site Vkontakte to take part in protests, with another 20,000 saying they might take part.
An hour before the protest was set to begin, small crowds were forming in an unauthorised spot in St Petersburg, reports the BBC's Richard Galpin - and scuffles had already broken out between demonstrators and police apparently attempting to make some arrests.
Authorities have granted permission for a demonstration in one location, but say protests anywhere else will be illegal and will be dealt with.
The official results of the elections to Russia's Duma showed that the ruling United Russia party saw its share of the vote fall from 64% to 49%, though it remained easily the biggest party.
But there is a widespread view, fuelled by mobile phone videos and accounts on internet social networking sites, that there was wholesale election fraud and that Mr Putin's party cheated its way to victory, our correspondent says.
On Friday, the presidential Council for Human Rights advising Mr Medvedev said the reports of vote-rigging were of deep concern, and that the elections should be rerun if they were confirmed.