This is a defining moment in Russia's 16-month full-scale war in Ukraine and potentially a challenge to Vladimir Putin's grip on power.
The Russian leader has accused Wagner mercenary boss Yevgeny Prigozhin of treason, embarking on an armed rebellion and "a stab in the back of our country".
One of Russia's most important figures, Prigozhin says his aim is "not a military coup but a march for justice".
What is happening with the Wagner group?
For months Prigozhin has played a vital role in Russia's military campaign in Ukraine, recruiting thousands to his Wagner mercenary group, especially from Russian jails.
He has long been in an open feud with the military chiefs running the war, but that has now turned into a revolt as they try to bring his forces under their command structure.
Wagner forces have crossed from occupied eastern Ukraine into the big southern Russian city of Rostov-on-Don, and claim to have taken control of the city and its military facilities.
President Putin says the situation is difficult but has promised to do everything to defend Russia.
Is this a coup?
All claims of a military coup are absurd, claims Prigozhin.
But what began as a no-holds-barred row over the military's failure to supply his mercenaries with sufficient kit and ammunition has now spilled over into a direct challenge to the two men in charge of prosecuting the war - Defence Minister Sergei Shoigu and armed forces chief Valery Gerasimov.
So far this is not a coup, as there has been no stated bid to seize power from the government. Prigozhin's "private military company" does not represent the military either, although he does claim to have widespread support in the armed forces.
But it is an attempt to topple Russia's top brass and therefore a challenge to the president's authority. And even though it was Russia's leader who allowed Prigozhin to develop his rival force, he clearly has no control of him any more.
The Kremlin is taking this extremely seriously. The whole Moscow region has been put on an alert under a strict "counter-terrorist operation regime" and major events have been cancelled.
Similar measures are in place in the Voronezh region near Ukraine's north-eastern border.
"There are 25,000 of us," Prigozhin claimed. "Everyone who wants, join us." That is not enough to threaten the president but it is a challenge to the military leadership.
Moving his forces across the border into Rostov, he claimed control of the military HQ from where the war is being run, and videos showed his men in the centre of the city apparently encountering no resistance.
He denies betraying Russia, and in an outspoken criticism of the president, says he is "deeply wrong".
What does Prigozhin want?
His idea of a "march for justice" is vague but his row with the military leadership has clearly escalated so fast he wants to force them out.
A video has emerged of Prigozhin telling a deputy defence minister and a general in Rostov on Friday that until the two main military figures come and talk to him, his mercenaries will block off the city and head for Moscow.
There are reports of a military helicopter being shot down and of a Wagner convoy heading north up the M4 motorway in the Voronezh region north of Rostov.
Prigozhin's quarrel is not with Russian soldiers in Ukraine, but with the "clowns" leading them, he argues. Several generals have appealed for him to calm down, but that seems too late.
What is the relationship between Putin and Prigozhin?
Prigozhin has long been a close ally of President Putin and has flourished under him, first as a wealthy businessman and then as a mercenary chief.
His Wagner fighters have died in big numbers in the bitter campaign to seize Bakhmut in eastern Ukraine, which lasted months and was never fully achieved. Prigozhin blamed the military top brass for shell shortages, with graphic videos and expletive-ridden social media rants exposing the failings and the fractures of Russia's military in Ukraine.
Until this point he never directly targeted his anger at the president. But his sarcastic references to "happy grandfather" were widely seen as indirect criticism. Last month he asked how Russia could win if it turned out that "this grandfather is a complete arsehole".
Earlier this month President Putin supported a move by Sergei Shoigu to have all mercenary groups in Ukraine sign defence ministry contracts by 1 July. Prigozhin has refused, seeing it as a challenge to his influence.
In a lengthy tirade on 23 June he told Russians that the whole justification for their war was a lie and merely an excuse for "a small group of scumbags" to promote themselves and deceive the public and president.
Events have moved very quickly since then.
Military sites in Rostov seized
Prigozhin accused the military of staging a deadly shelling on his men in Ukraine, but the military denied it and he failed to produce the kind of evidence he often does.
Late on Friday he announced his "march for justice" was under way. His force of 25,000 would be just a "tactical reserve" and the whole of the army and the whole country would be their strategic reserve.
Gen Sergei Surovikin, deputy commander of forces in Ukraine, appealed to him to step back and submit to President Putin's authority.
But by morning Prigozhin's men had reached Rostov: "We are inside [military] headquarters." He later said they had taken the city "without a shot".
Wagner fighters are also said to have seized military facilities in Voronezh, further north towards Moscow, and videos have shown a convoy of vehicles heading north on the M4 towards Moscow.
A serious moment for Putin and Russia
This is not a direct challenge to Russia's war in Ukraine and Prigozhin has maintained he is not disputing the president's leadership.
But it is sufficiently serious for the Russian leader to give a determined and uncompromising five-minute televised address.
Prigozhin has threatened to head towards Moscow if his military demands are not met. What happens if he follows through with that?
At the very least, Vladimir Putin looks like a leader who has lost control of a fast-moving situation.
Prigozhin has substantial public support in Russia and, even if his challenge does fall apart, this has become a moment of crisis for a military that has relied on his mercenaries in Ukraine.
It is also a defining moment for the Putin leadership and a wake-up call for Russians. It is too soon to say how it will end.
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