The rally was organized by a coalition of opposition groups, including chess-champion-turned-politician Garry Kasparov's liberal United Civil Front, radical writer Eduard Limonov's National Bolshevik Party, and the Russian People's Democratic Union, led by former pro-market prime minister Mikhail Kasyanov.
They accuse President Vladimir Putin and his associates of infringing on liberal freedoms and muzzling the opposition.
Some 1,500 activists gathered for the rally in Triumfalnaya Square Saturday afternoon amid tight security. The square was cordoned off by police, and several dozen military trucks blocked entry from nearby streets. A helicopter was hovering over the site as Kasparov, Kasyanov and other coalition leaders took turns to address the gathering from a speaker's platform. Protesters carried slogans reading "Bring Elections Back," "All Rise to Defend Constitution" and the like.
The initial plan involved a march along Moscow's main thoroughfare, Tverskaya Street, all the way to the Kremlin. But City Hall, citing traffic congestion concerns, banned the opposition groups from marching downtown and said only a rally would be permitted.
Most of the detained opposition activists are said to be members of the banned National Bolshevik Party.
Earlier, according to rally organizers, police reportedly apprehended a leader of the People's Democratic Union, Ivan Starikov. Law-enforcement officials, however, have neither confirmed nor denied that information so far.