Saturday's protest is being seen as a litmus test for the opposition. Mr Gorbachev has said he may speak at the rally and a clutch of famous journalists, writers, opposition leaders and bloggers are due to take to the podium. The Kremlin has sanctioned a rally of up to 50,000 people and the precise turnout will be closely watched to see whether the protest movement is gaining or losing momentum. Mr Putin is still favourite to return to the presidency after a March 4 vote but the protests have put him on the defensive, a position he is not used to being in.