Alexei Navalny called for the nationwide protests after he published reports claiming that Mr Medvedev controlled mansions, yachts and vineyards through a shadowy network of non-profit organisations.
Russian police declared the protest "illegal" and warned that they will "bear no responsibility for any possible negative consequences" for people who show up, but Navalny vowed to press ahead anyway. "Our issue today is the fight against corruption". They appeared to be one of the largest coordinated outpourings of political resentment in Russian Federation since the massive 2011-2013 rallies against a fraud-tainted parliamentary election.
Sunday's march in Moscow was one of the biggest unauthorised demonstrations in recent years, with police putting turnout at 7,000-8,000 people.
Russian authorities had warned Navalny's supporters on Friday not to attend the rally because the event had not been sanctioned by the city administration.
Elsewhere, at a rally in the far eastern city of Vladivostok, one media source reported seeing 30 people detained after unfurling banners reading "The prime minister should answer".
The detentions started after protesters unfurled banners reading "Corruption steals our future" and "The prime minister should answer".
Alexei Navalny, the anti-corruption campaigner who is leading the opposition to President Vladimir Putin, was arrested while walking from a nearby subway station to the demonstration, according to Associated Press journalists at the scene.
Witnesses said at least four people holding banners were detained on the city's Labour Square, where opposition protesters, nationalists and supporters of the pro-Kremlin United Russia party gathered.
"Corruption affects every person".
In Saint Petersburg, about 4,000 people gathered in the city centre.
Violence broke out during the demonstrations on Sunday, with many protesters accusing the police of using excessive force.
Navalny, a 40-year-old lawyer by training announced plans to run for the presidency after he won a surprise 27 percent of the vote in the Moscow mayoral election in 2013.