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.

Russia’s top investigative agency has filed criminal charges against tycoon and British newspaper backer Alexander Lebedev.

One of president Vladimir Putin’s fiercest critics has been charged with theft and is facing a 10-year prison sentence as the Kremlin steps up a crackdown on dissent.