What’s happening in Russia? Here’s what we know.

Russian generals on Friday accused a Russian mercenary tycoon of trying to mount a coup against President Vladimir V. Putin. It signaled an extraordinary open confrontation between the Wagner chief and the military, who have feuded for months over Russia’s war tactics in Ukraine.

There were reports overnight of military movements in an area of southern Russia near the border with Ukraine. And on Saturday morning, the tycoon, Yevgeny V. Prigozhin, claimed to have control of parts of the military command headquarters in southern Russia.

It was unclear how much the Wagner forces controlled or how much of a threat they posed to the Kremlin. But the confrontation already amounted to the biggest challenge to Mr. Putin’s authority since Russia invaded Ukraine 16 months ago.

In an address to the nation, Mr. Putin called the Wagner forces’ actions “a stab in the back of our country and our people” and vowed “decisive actions.”

Here’s what we know.

What’s happening?

Tension escalated late Friday after Mr. Prigozhin accused the Russian military of attacking his fighters’ encampments — a claim that could not be immediately verified. He also described the invasion of Ukraine as a “racket” perpetrated by a corrupt Russian elite.

Mr. Prigozhin vowed that what he said was his 25,000-strong mercenary force would go on the offensive against the Russian defense ministry, though he said that the actions were not a “military coup.”

The Russian authorities responded by charging Mr. Prigozhin with “organizing an armed rebellion.” A Russian general urged Mr. Prigozhin’s fighters not to “play into the hands” of an enemy that he said was waiting for Russia’s internal political situation to worsen.

Video footage showed armored vehicles from the Russian military in the southern Russian city of Rostov-on-Don, near the war’s front line in Ukraine where Mr. Prigozhin’s fighters had been operating.

Additional videos circulating online and verified by The New York Times showed dozens of soldiers getting out of military vehicles and pointing their guns at the compound that forms the military command post in southern Russia.

Early Saturday, the governor of the Rostov region asked residents to stay in their homes, saying that the authorities were “doing everything necessary” to ensure their safety. The governor of the nearby Voronezh region, north of Rostov, also said that a convoy of military equipment was moving along a local highway. It was not clear which direction it was moving.

As the events played out in Russia, Ukraine’s armed forces posted three words on Twitter: “We are watching.”

Who is Prigozhin?

The St. Petersburg tycoon has for years been part of a charmed circle of Russian oligarchs with close ties to President Putin. In 2018, he was one of 13 Russians indicted by a federal grand jury in the United States for interfering in the 2016 American election.

Mr. Prigozhin’s Wagner mercenary force, a shadowy private military company, first emerged during Russia’s illegal annexation of Crimea in 2014. It has since exerted influence on behalf of Moscow in Syria, Libya, the Central African Republic, Sudan, Mali and Mozambique.

Wagner is important to the Kremlin’s war in Ukraine and led the recent assault on the eastern Ukrainian city of Bakhmut. And Mr. Prigozhin, who has recruited fighters from prisons, has been widely seen as a symbol of wartime Russia: ruthless, shameless and lawless.

Why is Prigozhin angry?

Mr. Prigozhin has in recent months launched accusations at Russia’s military leadership. He blames Russian generals for failing to provide his forces with enough ammunition and for ignoring soldiers’ struggles.

The Kremlin tolerated his broadsides for months, even as some analysts said that Mr. Prigozhin was poised to turn his new prominence into broader political influence, possibly threatening Mr. Putin’s grip on power.

But official patience had clearly evaporated by Saturday morning, when the country’s prosecutor general announced that Mr. Prigozhin was being investigated on charges that carried a maximum prison term of 20 years. TASS, a Russian state news agency, reported that he had been charged.

Later on Saturday morning, Mr. Putin addressed the situation in Rostov-on-Don during a brief speech on state television, saying that “decisive actions” would be taken to stabilize it. He said the functioning of military and civilian institutions in the southern Russian city of a million people had “essentially been blocked.”

“Actions that divide our unity are in essence defeatism before one’s own people,” Mr. Putin said.