The head of the Wagner mercenary group said Saturday he had crossed into Russia with his forces to topple Moscow's military leadership, saying he and his 25,000 fighters were "ready to die".
"All of us are ready to die. All 25,000, and then another 25,000," Yevgeny Prigozhin, 62, said in an audio message, after earlier accusing the Russian top brass of launching strikes against his men.
"We are dying for the Russian people."
In response, Russian authorities said security had been tightened in several regions and the mayor of Moscow announced that "anti-terrorist" measures were being taken in the capital.
The FSB security service accused Prigozhin of attempting to launch a "civil conflict" and urged Wagner fighters to detain him.
"We will destroy everything that stands in our way," Prigozhin said earlier, in the most audacious challenge to President Vladimir Putin since the start of the offensive in Ukraine last year.
Prigozhin said his forces, who have spearheaded much of Russia's offensive in Ukraine, had entered the southern Russian region of Rostov, taken control of the city, and had also shot down a Russian military helicopter.
He did not, however, provide proof, and AFP could not independently verify the claims.
Authorities in Rostov urged residents to stay home.
"Law enforcement agencies are doing everything necessary to ensure the safety of residents," Rostov governor Vasily Golubev said.
Videos and pictures posted online, including by TASS, showed armed men surrounding administrative buildings in Rostov and tanks deployed in the city centre. It was not clear who the armed men were.
In Moscow, critical facilities were "under reinforced protection", the TASS state-run news agency reported, citing a law enforcement source.
Prosecutor General Igor Krasnov had informed Putin of "the initiation of a criminal case in connection with an attempt to organise an armed rebellion", Kremlin spokesman Dmitry Peskov said, adding the president was getting regular updates on the situation.
- Missile strikes -
The extraordinary developments came after Prigozhin accused Moscow of targeting his forces with missile strikes that he said killed "a huge number of our fighters".
"The council of commanders of PMC Wagner has made a decision -- the evil that the military leadership of the country brings must be stopped," he said in a series of furious audio messages released by his spokespeople.
He warned Russians against resisting his forces and called on them to join him.
"We need to put an end to this mess," he said, adding, "this is not a military coup, but a march of justice".
In a statement, the FSB said: "Prigozhin's statements and actions are in fact a call to start an armed civil conflict on the territory of the Russian Federation and a stab in the back to Russian servicemen fighting pro-fascist Ukrainian forces".
While Prigozhin's outfit has spearheaded much of Russia's offensive in Ukraine, he has in recent months engaged in a bitter feud with Moscow's military leadership and has repeatedly blamed Defense Minister Sergei Shoigu and Valery Gerasimov, chief of the general staff, for his fighters' deaths.
- 'Urge you to stop' -
The Russian defence ministry denied Prigozhin's claims of an attack on his forces, saying the statements "do not correspond to reality".
It later said Ukrainian troops were taking advantage of the infighting to ready an assault near the east Ukraine hotspot of Bakhmut.
A prominent Russian general urged Prigozhin to call off efforts to remove the defense ministry leadership.
"I urge you to stop," Sergei Surovikin, commander of Russia's aerospace forces, said in a highly unusual video address.
"Before it is too late, it is necessary... to obey the will and order of the popularly elected President of the Russian Federation".
Anti-Kremlin figure Mikhail Khodorkovsky, however, urged Russians to support Prigozhin, saying it was acceptable to back "even the devil" in taking on the Kremlin.
Washington-based think tank the Institute for the Study of War said the Wagner chief's attempt to force a leadership change in the defence ministry "is unlikely to succeed" given that Surovikin had denounced his call for rebellion.
Kyiv's defense ministry said it was monitoring the situation.
Ukraine was also on high alert after a fresh barrage of Russian missiles Saturday, with casualties and damage reported in Kyiv and the central city of Dnipro.
US President Joe Biden had been briefed on the situation in Russia and Washington "will be consulting with allies and partners on these developments", National Security Council spokesman Adam Hodge said.
On Friday, Prigozhin said Moscow's forces were retreating in Ukraine's east and south following the start of Kyiv's counteroffensive early this month. That directly contradicted Putin's account that Ukraine was suffering "catastrophic" losses and that there was a lull in fighting.
"We are washing ourselves in blood," Prigozhin said.
"No one is bringing reserves. What they tell us is the deepest deception," he added, referring to the Russian military and political leadership.
- Questioning military operation -
After years of operating in the shadows, Prigozhin has now admitted to running the elusive mercenary group Wagner and even interfering in US elections.
His forces, bolstered by tens of thousands of prison recruits, played a central role in Russia's capture of the town of Bakhmut in the eastern region of Donetsk, the longest and bloodiest battle of the conflict.
However, this week he accused Moscow's top brass of deceiving Russians about the offensive in Ukraine.
"Why did the special military operation begin?" he said. "The war was needed for the self-promotion of a bunch of bastards."
Prigozhin rose from a modest background to become part of the inner circle around Putin.
He spent nine years in prison in the final period of the USSR after being convicted of fraud and theft. In the chaos of the 1990s, he began a moderately successful business selling hot dogs.
From there he fell into the restaurant business and opened a luxury location in Saint Petersburg whose customers included Putin, then making the transition from working in the KGB to local politics.
In recent months, Prigozhin has become embroiled in a bitter power struggle with the defense ministry, accusing the military of attempting to "steal" his victories in Ukraine
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