MOSCOW, December 16 (RIA Novosti) - Some fifteen hundred supporters of a coalition of Russian opposition groups have gathered for a rally in central Moscow Saturday, with five times as many police called up to maintain security.

The coalition, which includes 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, has organized the rally to protest what it sees as the Kremlin's backsliding on democracy. It accuses President Vladimir Putin and his associates of infringing on liberal freedoms and muzzling the opposition.

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.

Kasparov, Kasyanov and other coalition leaders have addressed the gathering from a speaker's platform in Triumfalnaya Square, not far from the Kremlin. Some of the protesters are carrying slogans reading "Bring Elections Back" and "All Rise to Defend Constitution."

The square has been cordoned off by police, with several dozen military trucks blocking entry from nearby streets. A helicopter is hovering over the site.