Russian opposition figures: Sergei and Anastasia Udaltsov

The BBC looks at some of Russia's key opposition figures.

Sergei and Anastasia Udaltsov

They have recently become known as the "revolutionary couple" of Russian politics.

Born in 1977, Sergei Udaltsov is one of the leaders of the left-wing, unauthorised opposition - as opposed to the political parties officially registered by the authorities.

Anastasia Udaltsova, a year younger than her husband, was formerly his press secretary but became significant in the protest movement in December 2011 after he was arrested on the day of the parliamentary elections because of an unauthorised protest.

Sergei Udaltsov went on hunger strike while in custody. A speech at the second mass rally on 24 December was recorded and shown on big screens and it was not until early January that he was released from jail.

He has been involved in Russian politics since he established the radical left-wing Red Youth Avant-garde (AKM) in 1998. He later joined Left Front, an umbrella organisation for left-wing youth groups, and rose to become its informal leader.

In recent years he has organised and led numerous "Days of Rage" against the authorities, some of which were sanctioned but most were not. He has been arrested around 100 times.

Although he and his colleagues have tried to register an official political party, their application has been rejected several times. Now, he hopes to register the party under more liberal laws promised by outgoing President Dmitry Medvedev.

Sergei Udaltsov addressed protesters at an opposition rally on 6 May - the eve of President Vladimir Putin's inauguration - and was arrested by police who stormed the platform.