One of the main celebrations will be a sanctioned march in central Moscow from Pushkin Square to the Karl Marx statue outside the Bolshoi Theatre, where a rally will be held. The march, led by Communist Party leader Gennady Zyuganov, was set to start at 5:00 p.m. (14:00 GMT).
Olga Shklyarova, spokeswoman for the Interior Ministry's public order department, said: "Today, representatives of parties and public political movements will be holding around 140 mass events dedicated to the 91st anniversary of the October Revolution. Around 151,000 people are expected to take part."
More than 14,000 police officers will be deployed to ensure the rallies pass peacefully, she said.
The Russian Communist Party published a message on its website from Zyuganov congratulating Russians on the anniversary of the Revolution, and saying the financial crisis has demonstrated the failures of capitalism.
"Today, history itself reminds us of the rightness of the Great Russian Revolution. The system of global capitalism has been seized by a severe economic crisis. This collapse has not passed by Russia, which has been forced to become a part of global capitalism. The authorities' incantations about a 'save haven' and an 'island of stability' turned out to be an empty noise."
"The socialist alternative is insistently knocking at the door, and we will mark the Revolution holiday with optimism under red banners," he said.
The overthrow of the Russian government in Petrograd (St. Petersburg), on October 25, 1917 under the Julian calendar, was an official public holiday in Russia, celebrated on November 7, until Unity Day was introduced in 2005, and celebrated on November 4.
A mass Communist rally is also underway in Ukraine's capital, Kiev. Thousands of supporters of the Communist Party of Ukraine have been gathered on Independence Square since midday, waving banners with slogans such as "Down with [President Viktor] Yushchenko and the Oligarchs!"
A much smaller rival rally has been taking place across the square, led by the nationalist Freedom party, waving blue-and-yellow Ukrainian flags. A police cordon separates the groups.
