Who are other Russian dissidents besides the late Alexei Navalny?

The sudden death of Russian President Vladimir Putin's most formidable antagonist has left an open wound in Russia's political opposition.

Alexei Navalny, 47, was the Kremlin's best-known critic at home and abroad. Before he died in a penal colony Friday, the anti-corruption crusader, protest organizer and politician with an arch sense of humor became the subject of an award-winning documentary. His channels on YouTube had millions of subscribers.

Many of them nevertheless persisted in challenging Putin — organizing abroad, pushing for sanctions on Russia, supporting like-minded Russians in exile or continuing to speak out from behind bars.