Yevgeny Prigozhin, the bombastic chief of Russia's private mercenary group Wagner, appears to be falling out of favor fast in Moscow, after he unleashed a tirade against the Russian military on Friday and vowed to retaliate against its leaders.

Russia's domestic intelligence service, the FSB, has opened a criminal case against Prigozhin, accusing him of "calling for an armed rebellion." It also urged his own mercenaries to detain him.

Here's what you need to know.

What did Prigozhin do?

Prigozhin on Friday accused Russia's military of attacking a Wagner camp and killing a "huge amount" of his men. He vowed to retaliate with force, insinuating that his forces would "destroy" any resistance, including roadblocks and aircraft.

"There are 25,000 of us and we are going to find out why there is such chaos in the country," he said.

Prigozhin later rowed back on his threat, saying his criticism of the Russian military leadership was a "march of justice" and not a coup – but by that point he appears to have already crossed a line with the Kremlin.

Late on Friday, Prigozhin said his fighters had entered Russia's Rostov region and he even claimed his troops had downed a helicopter that fired on a "column" of civilians.

How did Russia respond?

Russia's Defense Ministry has denied attacking Wagner troops, calling the claim "informational propaganda." And the FSB also opened a criminal case against Prighozhin for his threats, accusing him of calling for "an armed rebellion."

"Prigozhin's statements and actions are in fact calls for the start of an armed civil conflict on the territory of the Russian Federation and are a stab in the back of Russian servicemen fighting pro-fascist Ukrainian forces," an FSB statement said, calling for Wagner fighters to detain their leader.

Russian President Vladimir Putin is aware of the situation, according to Kremlin spokesman Dimitry Peskov.

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Russian officials meanwhile appeared to take no chances with security measures stepping up in Moscow, according to Russian state media TASS.

Social media posts showed military vehicles were seen driving around the main streets of the Russian capital in the early hours of Saturday.

Who is Prigozhin?

Prigozhin has known Putin since the 1990s. He became a wealthy oligarch by winning lucrative catering contracts with the Kremlin, earning him the moniker "Putin's chef".

His transformation into a brutal warlord came in the aftermath of the 2014 Russian-backed separatist movements in the Donbas in eastern Ukraine.

Prigozhin founded Wagner to be a shadowy mercenary outfit that fought both in eastern Ukraine and, increasingly, for Russian-backed causes around the world.

CNN has tracked Wagner mercenaries in the Central African Republic, Sudan, Libya, Mozambique, Ukraine and Syria. Over the years they have developed a particularly gruesome reputation and have been linked to various human rights abuses.

Prigozhin's political star rocketed in Russia after Moscow's full invasion of Ukraine in February 2022.

While many regular Russian troops saw setbacks on the battlefield, Wagner fighters seemed to be the only ones capable of delivering tangible progress.

Known for its disregard for the lives of its own soldiers, the Wagner group's brutal and often lawless tactics are believed to have resulted in high numbers of casualties, as new recruits are sent into battle with little formal training – a process described by retired United States Lt. Gen. Mark Hertling as "like feeding meat to a meat grinder."

Prigozhin has used social media to lobby for what he wants and often feuded with Russia's military leadership, casting himself as competent and ruthless in contrast to the military establishment.

His disagreements with Russia's top brass exploded into the public domain during the grim and relentless battle for Bakhmut during which he repeatedly accused the military leadership of failing to supply his troops with enough ammunition.

In one particularly grim video from early May, Prigozhin stood next to a pile of dead Wagner fighters and took aim specifically at Russian Defense Minister Sergei Shoigu and chief of the Russian armed forces Gen. Valery Gerasimov.

"The blood is still fresh," he says, pointing to the bodies behind him. "They came here as volunteers and are dying so you can sit like fat cats in your luxury offices."

Putin presides over what is often described as a court system, where infighting and competition among elites is in fact encouraged to produce results, as long as the "vertical of power" remains loyal to and answers to the head of state.

But Prigozhin's increasingly outrageous outbursts have sparked speculation in recent weeks that even he could be going too far.