Forces of law and order were out in strength

Organisers, including former chess champion Garry Kasparov, had hoped up to 5,000 people would attend. But turnout was about 2,000.

Some witnesses said their numbers were dwarfed by the police and security service presence in central Moscow.

A police helicopter flew overhead, threatening to drown out chants.

The demonstration comes 15 months before the next presidential election.

Although Mr Putin is not eligible to stand, analysts believe his popularity is such that whoever he backs is assured of an easy victory.

The 2,000 or so protesters were kept confined behind barriers and surrounded by helmeted riot police, says the BBC's James Rodgers in Moscow.

Today we finally came together and called for our collaborators and supporters that we start our political process to change this particular power Mikhail Kasyanov Former PM

Some reports suggested thousands of riot police and soldiers were on the streets for the protest.

The anti-Putin lobby ranged from radical youth groups to communists nostalgic for the Soviet Union to free marketeers.

Former Russian Prime Minister Mikhail Kasyanov - now a Kremlin opponent - insisted this was a significant and important day.

"Today we finally came together and called for our collaborators and supporters that we start our political process to change this particular power. Fight together with us, politically, and get prepared for elections.

Mr Kasparov left professional chess to devote his attention to opposition politics and now runs an organisation called the United Civil Front.

The Russian opposition has long been wracked by internal divisions, the BBC regional analyst Stephen Eke says.

But he adds that with parliamentary elections due next year, and presidential elections due early in 2008, they have begun to co-ordinate their efforts.