Former separatist commander and nationalist military blogger Igor Girkin, a frequent critic of Russia's leadership, was arrested on Friday and remanded in custody awaiting trial on charges of extremism.
Girkin -- better known by his alias Igor Strelkov -- is a supporter of the Ukraine offensive but also regularly criticised its conduct on the messaging app Telegram, where he was followed by around 875,000 people.
By opening a case against him, observers said authorities were sending a signal that any criticism -- even from supporters of the military operation in Ukraine -- are off-limits in the aftermath of the Wagner mercenary group's short-lived rebellion.
A Moscow court ordered Girkin to be placed in detention pending trial on charges of "public calls for extremism" that could land him in prison for five years.
"Justice in our country, once again, has not triumphed" his lawyer Alexander Molokhov told reporters outside the Meshchansky Court in Moscow.
Molokhov said the defence would appeal the court's decision and explained the case against his client was based on two social media posts in which Girkin discussed Russian-annexed Crimea and army supplies.
Several dozen supporters gathered around the court, with at least one detained.
"The repressive machine has gone into full gear, this is an excuse to knock him out from public work," said Pavel Gubarev, head of the "Club of Angry Patriots" that Girkin co-founded.
Shortly before being detained, Gubarev said Girkin's supporters would fight using all legal means.
"Igor Ivanovich Strelkov isn't just not an extremist: he is a patriot who has gone through five wars in the interests of Russia."
A former FSB colonel, Girkin was one of the key figures in the pro-Kremlin insurgency when fighting broke out in eastern Ukraine in 2014.
Girkin ruled the then-rebel stronghold of Sloviansk with an iron fist, with executions for petty theft reportedly carried out under his rule.
But he was squeezed out of the separatist leadership later that year under mysterious circumstances and returned to Russia, where he lost all influence, until the offensive began.
In 2022, he was one of three men sentenced by a Dutch court to life imprisonment over the downing of Malaysia Airlines flight MH17 over Ukraine in 2014.
Girkin came back to the spotlight after the beginning of the offensive, becoming one of the most vocal critics of Putin and of the way in which Russia's offensive in Ukraine has been conducted.
In one of his most recent posts Girkin urged Putin to hand over power to a successor.
"The country will not survive another six years of this cowardly mediocrity in power," he wrote on Telegram.
Independent political analyst Tatiana Stanovaya said Girkin had "long crossed all possible red lines".
But his detention comes around a month after the attempted mutiny by the mercenary group Wagner led by Yevgeny Prigozhin, from which the Kremlin emerged visibly weakened.
Prigozhin was the most strident voice for those inside Russia criticising failures in Russia's campaign and the strategies used by Defence Minister Sergei Shoigu and army chief Valery Gerasimov.
Girkin denounced Prigozhin's mutiny but kept on criticising the incompetence of Russia's conventional military commanders.
The case against Girkin "is one of the consequences of Prigozhin's rebellion: the army received more political opportunities to suppress its opponents in the public space," Stanovaya said.
She did not expect mass arrests but said: "The most radical ones may be prosecuted, so that the rest will be more careful."
Criticism of Russia's assault on Ukraine has been outlawed and all key liberal opposition figures are either behind bars or in exile.
"The authorities decided that uncontrolled ultra-patriots are no less dangerous than the extra-systemic liberal opposition," said political analyst Ilya Gerashchenkov.
