Yevgeny Roizman, a prominent Kremlin critic and popular former mayor, stood trial on Wednesday over accusations of “discrediting” the Russian military.
Roizman is one of just a few leading opposition figures who remain in Russia and have not been jailed. He faces up to five years in prison.
Since invading Ukraine in February last year, the Russian authorities have intensified their crackdown on domestic critics, with almost all of the Kremlin's major opponents in exile or behind bars.
In 2013, Roizman became Russia's most prominent opposition mayor, serving for five years in public office in the Urals city of Yekaterinburg.
He resigned from his post after authorities moved to scrap mayoral elections, but he has been closely involved in charity work since then.
Until recently, Roizman appeared largely untouchable.
But in August, the authorities opened a criminal probe against him for "discrediting" the Russian army in comments he made about the invasion of Ukraine.
Russia's Justice Ministry placed him on its list of "foreign agents" back in November.
Dressed in blue jeans and a white T-shirt, Roizman pleaded not guilty at the start of the trial in Yekaterinburg Wednesday.
Roizman, who openly denounces President Vladimir Putin and the war, has said he knows he could go to prison at any moment.
The former politician has a penchant for crude language and has peppered Twitter with swear words to mock officials in Russia, much to the delight of his supporters.
Tall and sporty, Roizman first rose to prominence in the early 2000s as an anti-drugs activist fighting Russia's severe narcotics epidemic.