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.

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.

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 Prighozhin?

Prigozhin, who has known Putin since the 1990s, saw his political star rise in Russia during the Ukraine war, as his fighters seemed to be the only ones capable of delivering tangible battlefield progress.

The Wagner boss 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.

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.