(Updates below the fold)
Too early to determine how far this mutiny will spread, but as of today 25,000 Russian Wagner mercenaries commanded by Yevgeny Prigozhin are driving for Moscow. Putin has bolstered the civil defense.

Russia has opened a criminal case against the head of the Wagner mercenary group for alleged armed mutiny, with security tightened in the capital.

Yevgeny Prigozhin accused the country's defence ministry of targeting his troops and ordering a rocket strike on Wagner's field camps in Ukraine - killing "a huge number of our comrades".

He warned that his troops will move to punish defence minister Sergei Shoigu - and urged the army not to offer resistance.

"This is not an armed rebellion, but a march of justice," Prigozhin said.

For those unfamiliar with Prigozhin:

Sergei Markov, a pro-Kremlin political commentator, described Prigozhin as "the second-most popular man after Putin" and a "symbol of Russia's military victory for millions of people"

Though I'm loathe to source The Guardian, they have this quote by Prigozhin I find interesting.

“This is not a military coup, this is a march of justice. Our actions do not hinder the armed forces in any way,” the Wagner chief said, adding that the “majority of soldiers” were on his side. The warlord has been arguing with top military officials for months, singling out Shoigu, over battlefield failures.

The question I have is: when Prigozhin says the majority of soldiers are on his side, does he mean Wagner's soldiers or the entire Russian army. The answer to that question may determine the seriousness of this developing situation.

Background on the clash between Shoigu and Prigozhin: A few weeks ago, Shoigu ordered all mercenaries to sign contracts with his office by the end of the monthy. Prigozhin refused. Putin sided with Shoigu.

“Wagner will not sign any contracts with Shoigu,” Prigozhin said in response, criticising Shoigu for failing to “properly manage military formations”.

When asked to comment on Putin’s statements on Wednesday, a defiant Prigozhin doubled down. “When we started participating in this war, no one said that we would be obliged to conclude agreements with the Ministry of Defence,” he said in a rare direct rebuke of the president.

“None of the Wagner fighters is ready to go down the path of shame again. And so no one will sign contracts,” he said, adding that he believed the Russian president would find a “compromise” for Wagner.

Putin had previously praised Prigozhin, who has become a popular figure among those unhappy with how the war has progressed so far.

“At the start of the special military operation, we quickly realised that the ‘carpet generals’ […] are not effective, to put it mildly,” Putin said. “People started to come out of the shadows who we hadn’t heard or seen before, and they turned out to be very effective and made themselves useful.”

Will update as more becomes clear.

UPDATE: Aljazeera saying Wagner forces have control of Rostov Army HQ.

Wagner chief Yevgeny Prigozhin has demanded Defence Minister Sergei Shoigu and Russia’s top general Valery Gerasimov come to meet him in the southern city of Rostov-on-Don.

In a video posted on social media, Prigozhin said he was now at the headquarters of the Southern Military District, which is in Rostov-on-Don.

“We have arrived here, we want to receive the chief of the general staff and Shoigu,” Prigozhin said on the video.

“Unless they come, we’ll be here, we’ll blockade the city of Rostov and head for Moscow.”

You can watch the video here.

UPDATE:
Putin about to address the situation live.

UPDATE: Wagner forces claim to have downed a Russian helo.

UPDATE: Medhurst declares this is not a coup.

UPDATE: Per Aljazeera,

Wagner chief Prigozhin says his fighters, who were advancing towards Moscow, were turning around and returning to their bases in order to avoid bloodshed.

In an audio message released by his press service, Prigozhin said, “They wanted to disband the Wagner military company. We embarked on a march of justice on June 23. In 24 hours we got to within 200 km [125 miles] of Moscow. In this time, we did not spill a single drop of our fighters’ blood.”

“Now, the moment has come when blood could be spilled. Understanding responsibility [for the chance] that Russian blood will be spilled on one side, we are turning our columns around and going back to field camps as planned.”

It sounds almost as if the Russians were coordinating the entire thing, so prepared were they.

So even if the Russian officialdom told themselves Prigozhin could be contained, the rousing of Ukrainian opponents in Russia might be another matter. Even with Prigozhin going rogue, the Western media seemed a bit surprised as to how Russia swung into high alert mode.

I had wondered if Prigozhin was merely bait so the Russians could root out the sleeper cells waiting to strike should Russian forces revolt. Staging a phoney mutiny would activate electronic activity. Per Naked Capitalism:

However, recall also the West has been talking ad nauseam about an uprising versus Putin. So whether by action or design, Prigozhin’s move will serve as a trigger for Ukraine sleeper cells in Russia to swing into action. And again, Rybar suggested the Russian security forces had been put on alert for that type of operation BEFORE the Prigozhin gambit:

At 2am June 22nd...information was received about the preparation of an armed attack on government facilities.

It goes on to state that FSB officers captured two GUR agents and extracted the identities of the entire chain of underground cells in the Moscow region. They began busily rounding up those people.

It will be interesting to see if charges against Prigozhin are dropped. That would indicate he was part of a Russian psyop to uncover Ukranian cells hiding near Moscow. If they go ahead and execute him, it would mean he only turned back when he realized the planned coup had already failed.