In a new sign of a widening crackdown on Russian opposition, a criminal investigation has begun into leftist leader Sergei Udaltsov and several other opposition activists for allegedly plotting mass riots.

Russian President Vladimir Putin met the Belorussian leader tonight, starting a visit that was closely watched for signs the two ex-Soviet neighbours were advancing towards a long-discussed merger.

Russian Communist Party chief Gennady Zyuganov today became the first prominent politician to throw his hat in the ring for the 2008 presidential election, a poll in which President Vladimir Putin is constitutionally barred from running.