Russian police say 25,000 people are in Moscow’s Bolotnaya Square for the rally against what they believe was a fraudulent election last Sunday.
Authorities have given permission for 30,000 to attend, and many more people are making their way there.
The protests are aimed at the ruling United Russia Party and prime minister Vladimir Putin, who will next year aim for the presidency.
Even though the nationwide protests are being called the biggest in twenty years, opposition is not universal.
One man criticised the thirty thousand police in Moscow, accusing them of protecting the government, not the people.
However, another woman said she did not think the regional areas of Russia were being taken into account, where she believed more people support Putin’s United Russia Party.
According a Russian news website, at least one person has been arrested at a similar demonstration in St Petersburg, where reports suggest more than 500 people have gathered.
Demonstrations – both sanctioned and banned – are believed to be taking place in at least 14 cities, in the first countrywide protests permitted in Putin’s era.