Putin strikes a tough tone in his first address since the uprising started.

Videos circulating widely on social media showed that military and national guard armored vehicles had been deployed in Moscow and the southern city of Rostov-on-Don, near the front line in Ukraine where Mr. Prigozhin’s fighters had been operating.

Mr. Prigozhin on Friday accused the Russian military of attacking his Wagner mercenary forces and, in a series of recordings posted to social media, pledged that his fighters would retaliate. Russian authorities, in turn, accused Mr. Prigozhin — whose broadsides against the Russian Defense Ministry had been tolerated by Mr. Putin for months — of trying to foment a revolt.

“This is a stab in the back of the country and the president,” Gen. Vladimir Alekseyev, the deputy head of Russia’s military intelligence agency, said in a video appeal to Mr. Prigozhin’s fighters, urging them to call off any rebellion. “This is a coup.”

Mr. Prigozhin’s Wagner mercenary force has proved pivotal to the Russian war effort in Ukraine, but in recent months, he repeatedly chastised Russia’s top brass for alleged corruption and indifference to regular soldiers’ lives. On Friday night, he took his accusations to a new level, claiming that the Russian military had attacked Wagner encampments, killing “a huge number of fighters.”

“The evil borne by the country’s military leadership must be stopped,” Mr. Prigozhin (pronounced pree-GOH-zhin) said in one of a series of voice recordings posted to the Telegram social network after 9 p.m. Moscow time.

Minutes later, he suggested that his Wagner mercenary force was prepared to go on the offensive against Russia’s own Defense Ministry, saying, “There’s 25,000 of us, and we are going to figure out why chaos is happening in the country.”

He denied that the actions were a “military coup.”

“This is a march for justice,” he said in another audio message on Telegram. “Our actions aren’t interfering with the troops in any way.”

Just past midnight Moscow time, Russia’s prosecutor general announced that Mr. Prigozhin was being investigated “on suspicion of organizing an armed rebellion” and would face as much as 20 years in prison if prosecuted.

The Wagner leader then defiantly took to Telegram again, saying his fighters were approaching the city of Rostov-on-Don and adding: “We are going farther. We will go to the end.”

Mr. Prigozhin’s whereabouts remained unclear, and there was no immediate confirmation that his forces were actually approaching the city.

While President Volodymyr Zelensky of Ukraine had yet to comment as of Friday night, one of his advisers, Mykhailo Podolyak, warned on Twitter that “tumultuous times are coming” for Russia.

White House officials said they were following the events, but would not say much more. “We are monitoring the situation and will be consulting with allies and partners on these developments,” said Adam Hodge, a National Security Council spokesman.

Mr. Prigozhin, a St. Petersburg restaurateur who leveraged his personal connections with Mr. Putin into lucrative government contracts, gained international prominence after his online “troll factory” interfered in the 2016 American presidential election — and after his Wagner fighters were deployed in Syria and across Africa as a shadow force believed to be fighting for Kremlin interests.

For months the Russian war effort has been hampered by the bitter feud between Mr. Prigozhin and top military leaders, whom he has accused in scathing terms of incompetence in conducting the war. He has asserted that Russia’s top brass have refused to provide Wagner forces with needed ammunition even as they fought alongside the Russian military for control of the Ukrainian city of Bakhmut.

But never before had Mr. Prigozhin accused Russia’s military leaders of attacking his forces, nor asserted in such stark terms that the Kremlin’s stated justification for the war was nonsense.

In a 30-minute video released on Friday, Mr. Prigozhin had described his country’s invasion of Ukraine as a “racket” perpetrated by a corrupt elite chasing money and glory without concern for Russian lives.

He also accused the Russian minister of defense, Sergei K. Shoigu, of orchestrating a deadly attack with missiles and helicopters on camps to the rear of the Russian lines in Ukraine, where his soldiers of fortune were bivouacked. And he accused Mr. Shoigu of overseeing the strikes himself from the town of Rostov-on-Don in southern Russia, near Ukraine.

The mercenary leader’s claims could not be immediately verified. The Russian defense ministry denied the allegations, saying in a statement that the messages Mr. Prigozhin had posted about supposed strikes on Wagner camps “do not correspond to reality.”

Dmitri S. Peskov, the Kremlin’s spokesman, said that Mr. Putin was “aware of all events around Prigozhin,” according to Interfax, a Russian news agency.

Mr. Prigozhin’s accusations created a ripple effect among Russian pro-war activists, who fear that an open conflict between the army and Wagner forces could threaten the Russian front lines during the Ukrainian counteroffensive. In Ukraine, some viewed his statements as more evidence of internal divisions within the Russian war effort.

In an earlier videotaped speech, Mr. Prigozhin did not explicitly impugn Mr. Putin, instead casting him as a leader being misled by his officials. But in dismissing the Kremlin’s narrative that the invasion was a necessity for the Russian nation, Mr. Prigozhin went further than anyone in Russia’s security establishment in publicly challenging the wisdom of the war.

“The war wasn’t needed to return Russian citizens to our bosom, nor to demilitarize or denazify Ukraine,” Mr. Prigozhin said, referring to Mr. Putin’s initial justifications for the war. “The war was needed so that a bunch of animals could simply exult in glory.”

Friday’s diatribes deepened the enigma of Mr. Prigozhin’s ambiguous role in Mr. Putin’s system. His Wagner troops, composed of veteran fighters as well as thousands of convicts whom Mr. Prigozhin personally recruited from Russian prisons, proved key in capturing the Ukrainian city of Bakhmut in May after a monthslong battle.

But, during the battle for Bakhmut, Mr. Prigozhin also emerged as a populist political figure, excoriating Russia’s military leadership for corruption. His angry recordings and videos posted to the Telegram messaging network cast top military and Kremlin officials as unaware and uncaring of the struggles of regular Russian soldiers.

So far, Mr. Putin has not reined in Mr. Prigozhin, even as Mr. Putin’s security forces have jailed or fined thousands of Russians for criticizing the military or opposing the war. Some people who know Mr. Putin have said they believe that he still sees Mr. Prigozhin as a loyal servant applying needed pressure on a sprawling military apparatus. Others theorized that the Kremlin had orchestrated Mr. Prigozhin’s tirades against Mr. Shoigu, the defense minister, to deflect blame from Mr. Putin himself.

But Friday’s statements complicated the picture, with Mr. Prigozhin going after not just Mr. Shoigu but also unnamed “oligarchs” around Mr. Putin, while casting the entire official rhetoric around the invasion as a sham. He said there was “nothing out of the ordinary” in Ukraine’s military posture on the eve of the February 2022 invasion — challenging the Kremlin’s justification that Ukraine was on the verge of attacking Russian-backed separatist territory in Ukraine’s east.

“Our holy war with those who offend the Russian people, with those who are trying to humiliate them, has turned into a racket,” he said.

Mr. Prigozhin also asserted in his video that Ukraine’s counteroffensive to gain back territory was going much more poorly for Russia than the government was letting on. On Telegram, pro-war commentators quickly pushed back against that claim, including Igor Girkin, a former paramilitary commander who himself has often criticized Russia’s top brass.

“Prigozhin already should have been handed over to a military tribunal for many things,” Mr. Girkin wrote. “Now also for treason.”

Julian E. Barnes and Cassandra Vinograd contributed reporting.