"They have forbidden us from discussing Putin", Mr Nemtsov, who plans to run for the country's presidency in March 2008, was cited as saying by Reuters.

The crackdown followed another anti-Putin march of 3,000 in Moscow, which saw the arrest of opposition leader and former world chess champion Garry Kasparov on Saturday. He was sentenced to five days in jail.

The move was immediately condemned by Washington, with the National Security Council issuing a statement describing the latest developments as "aggressive tactics used by Russian authorities against opposition protesters".

"We are troubled that Garry Kasparov and other leaders of the opposition have been arrested and detained. We call for them to be given proper access to legal counsel and fair treatment in processing", the statement says.

According to the leader of the centre-right group in the European Parliament, Joseph Daul, Russian authorities are losing their credibility on the international scene by preventing the opposition from exercising its fundamental right to demonstrate.

"Our group demands the European Union to condemn such an attitude and demands the immediate release of supporters of the opposition group, The Other Russia", Mr Daul added.

Parliamentary elections are scheduled to take place on 2 December 2007, followed by presidential race in March 2008.

Vladimir Putin's party, United Russia, is expected to win a landslide victory in the poll, with Mr Putin likely to take the position of a future prime minister.

The country's constitution bars him from seeking a third presidential term.