By Paul Kirby - BBC News
The latest spat between the Wagner mercenary group and the Kremlin marks an important moment in Russia's 16-month all-out war in Ukraine and potentially poses a challenge to Vladimir Putin's grip on the country.
Yevgeny Prigozhin, the leader of the Wagner mercenary group, has been charged by the Russian leader with treason, inciting an armed rebellion and treason.
Prigozhin, a prominent figure in Russia, insists his objective is not a military coup, but a pursuit of justice. For months, he has played a crucial role in Russia's military campaign in Ukraine, recruiting numerous individuals, particularly from Russian prisons, into his Wagner mercenary group. However, his relationship with the military leaders overseeing the war deteriorated, leading to a full-scale revolt.
Wagner forces have moved from occupied eastern Ukraine to Rostov-on-Don, a large city in southern Russia, and claim to have gained control of military installations in the area. President Putin acknowledges the challenging situation, but has vowed to do whatever is necessary to protect Russia.
So is this considered a coup?
Prigozhin dismisses any claims of a military coup as absurd.
However, what initially began as a heated dispute over the army's failure to adequately equip and supply its mercenaries has now escalated into a direct challenge against Defense Minister Sergei Shoigu and Chief of the General Staff Valery Gerasimov - two the main figures responsible for the prosecution of the war.
Currently, it cannot be classified as a coup since there was no attempt to take control of the government by force.
Prigozhin's self-proclaimed "private military company" does not represent the official Russian military, despite his claims of widespread support.
However, it poses a challenge to the authority of Russia's top leadership as it seeks to undermine and replace them. Although Prigozhin was initially allowed by the Russian leader to develop his rival force, it is clear that the leader no longer has control over him.
The Kremlin is treating this situation very seriously. The entire Moscow region has been placed on high alert, operating under a "strict anti-terrorist operation regime," and major events have been canceled as a precaution. Similar measures have been implemented in the Voronezh region near the northeastern border of Ukraine.
Prigozhin claims: "We are 25,000 people. Anyone who wants can join us." While this number is not enough to pose a direct threat to the president, it does challenge the military leadership.
By moving his forces across the border to Rostov, Prigozhin appears to have surrounded the military headquarters responsible for managing the war effort. He claims that both the defense minister and the chief of staff have escaped.
Prigozhin denies betraying Russia and openly criticizes the president, saying he is "deeply mistaken".
What are Prigozhin's goals?
His call for a "march for justice" lacks clarity, but his conflict with the military leadership has escalated rapidly, indicating his intention to force them out.
A video has emerged showing Prigozhin demanding that a deputy defense minister and a general come to Rostov to negotiate with him. He states that until these two senior military figures engage in dialogue, his mercenaries will blockade the city and head towards Moscow.
There were reports of a military helicopter downing, as well as a Wagner convoy heading north on the M4 highway in the Voronezh region, north of Rostov.
Prigozhin's quarrel is not with Russian soldiers in Ukraine, but with military leaders whom he considers incompetent. Although several generals have urged him to calm down, it seems to be too late.
What is the relationship between Putin and Prigozhin?
Prigozhin has long been a close ally of President Putin and flourished under his leadership, first as a wealthy businessman and later as the leader of a mercenary group.
Who are the mercenaries of Russia's Wagner Group? Prigozhin's Wagner fighters have suffered heavy casualties in their protracted and unsuccessful campaign to capture Bakhmut in eastern Ukraine. Prigozhin blamed senior military leaders for insufficient supplies, and he publicly exposed the Russian military's internal failures and divisions in Ukraine through graphic videos and profanity-laden social media rants.
While Prigozhin never directly criticized the president, his sarcastic references to a "happy grandfather" were widely interpreted as indirect criticism.
Military facilities in Rostov have been seized
Prigozhin accused the military of orchestrating a deadly bombing of his men in Ukraine, but the military denied it and he failed to produce the kind of evidence he often does.
Late Friday he announced that his "march for justice" was underway. His force of 25,000 men would only be a "tactical reserve" and the entire army and the entire country would be their strategic reserve.
General Sergei Surovikin, deputy commander of forces in Ukraine, called on him to step down and submit to President Putin's authority.
But by morning Prigozhin's men had reached Rostov: "We are inside the [military] headquarters."

A serious moment for Putin and Russia
This is not a direct challenge to Russia's war in Ukraine or the president's leadership. But it is serious enough for the Russian leader to deliver a firm and uncompromising five-minute televised speech.
Prigozhin has threatened not only to set up camp in Rostov, but to march on Moscow if his military demands are not met.
So far he has only fought with the military leadership to increase arms supplies, now he is taking the leadership himself.
Prigozhin has considerable public support in Russia and, even if his challenge fails, this is at least a moment of crisis for an army that has relied on mercenaries in Ukraine.
But this is also a defining moment for Putin's leadership and a wake-up call for Russians. It's too soon to tell how it will end.