By Paul Kirby
BBC News
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.
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Wagner mercenaries seize Russian city's military HQ
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.