Updated June 23, 2023 at 9:26 PM ET

The owner of the Wagner private military contractor made his most direct challenge to the Kremlin yet on Friday, calling for an armed rebellion aimed at ousting Russia's defense minister. The security services reacted immediately by calling for the arrest of Yevgeny Prigozhin and opening a criminal investigation.

In a sign of how seriously the Kremlin was taking the threat, security was heightened in Moscow and in Rostov-on-Don, which is home to the Russian military headquarters for the southern region and also oversees the fighting in Ukraine.

While the outcome of the confrontation was still unclear, it appeared likely to further hinder Moscow's war effort as Kyiv's forces were probing Russian defenses in the initial stages of a counteroffensive.

Prigozhin claimed early Saturday that his forces had crossed into Russia from Ukraine and had reached Rostov, saying they faced no resistance from young conscripts at checkpoints and that his forces "aren't fighting against children."

"But we will destroy anyone who stands in our way," he said in one of a series of angry video and audio recordings posted on social media beginning late Friday. "We are moving forward and will go until the end."

He claimed that the chief of the General Staff, Gen. Valery Gerasimov, scrambled warplanes to strike Wagner's convoys, which were driving alongside ordinary vehicles.

Despite Prigozhin's statements that Wagner convoys had entered Rostov-on-Don, there was no confirmation of that yet on Russian social networks. Videos showed heavy trucks blocking highways leading to the city, long convoys of National Guard trucks were seen on a road outside Rostov-on-Don and armored vehicles were roaming the streets.

Prigozhin said Wagner field camps in Ukraine were struck by rockets, helicopter gunships and artillery fire on orders from Gerasimov following a meeting with Defense Minister Sergei Shoigu, at which they decided to destroy Wagner.

The Wagner forces have played a crucial role in Russia's war in Ukraine, succeeding in taking the city where the bloodiest and longest battles have taken place, Bakhmut. But Prigozhin has increasingly criticized Russia's military brass, accusing it of incompetence and of starving his troops of weapons and ammunition.

Prigozhin said late Friday his troops would punish Shoigu in an armed rebellion and urged the army not to offer resistance. "This is not a military coup, but a march of justice," Prigozhin declared.

The National Anti-Terrorism Committee, which is part of the Federal Security Services, or FSB, has charged him with calling for an armed rebellion, which carries a penalty of up to 20 years in prison.

The FSB urged Wagner's contract soldiers to arrest Prigozhin and refuse to follow his "criminal and treacherous orders." It called his statements a "stab in the back to Russian troops" and said they amounted to fomenting an armed conflict in Russia.

President Vladimir Putin has been informed about the situation and "all the necessary measures were being taken," Kremlin spokesman Dmitry Peskov said.

Heavy military trucks and armored vehicles were seen in several parts of central Moscow early Saturday, and soldiers toting assault rifles were deployed outside the main building of the Defense Ministry. The area around the presidential administration near Red Square was blocked, snarling traffic.

But even amid the heightened military presence, downtown bars and restaurants were filled with customers. At one club near the headquarters of the FSB, people were dancing in the street near the entrance.

Prigozhin, whose feud with the Defense Ministry dates back years, had refused to comply with a requirement that military contractors sign contracts with the ministry before July 1. In a statement late Friday, he said he was ready to find a compromise but "they have treacherously cheated us."

"Today they carried out a rocket strike on our rear camps, and a huge number of our comrades got killed," he said. The Defense Ministry denied attacking the Wagner camps.

Prigozhin claimed that Shoigu went to the Russian military headquarters in Rostov-on-Don personally to direct the strike and then "cowardly" fled.

"This scum will be stopped," he said of Shoigu.

"The evil embodied by the country's military leadership must be stopped," he shouted, urging the army not to offer any resistance to Wagner as it moves to "restore justice."

Col. Gen. Sergei Surovikin, the deputy commander of the Russian group of forces fighting in Ukraine, urged the Wagner forces to stop any move against the army, saying it would play into the hands of Russia's enemies, who are "waiting to see the exacerbation of our domestic political situation."

Tatiana Stanovaya, a political analyst, predicted this would be the end of Prigozhin.

"Now that the state has actively engaged, there's no turning back," she tweeted. "The termination of Prigozhin and Wagner is imminent. The only possibility now is absolute obliteration, with the degree of resistance from the Wagner group being the only variable. Surovikin was dispatched to convince them to surrender. Confrontation seems totally futile."

Lt. Gen. Vladimir Alexeyev, a top military officer, denounced Prigozhin's move as "madness" that threatened to unleash a civil war.

"It's a stab in the back to the country and the president," he said. "It's impossible to imagine a stronger blow to the image of Russia and its armed forces. Such a provocation could only be staged by enemies of Russia."

The Defense Ministry said in a statement that Ukraine's military was concentrating troops to launch an attack around Bakhmut to take advantage of "Prigozhin's provocation." It said Russian artillery and warplanes were firing on Ukrainian forces as they prepared to start an offensive in the area.

In other developments in the Ukraine war, Ukrainian President Volodymyr Zelenskyy called on other countries to heed warnings that Russia may be planning to attack an occupied nuclear power plant to cause a radiation disaster.

Members of his government briefed international representatives on the possible threat to the Zaporizhzhia Nuclear Power Plant, whose six reactors have been shut down for months. Zelenskyy said he expected other nations to "give appropriate signals and exert pressure" on Moscow.

The Kremlin's spokesman has denied the threat to the plant is coming from Russian forces.

The potential for a life-threatening release of radiation has been a concern since Russian troops invaded Ukraine last year and seized the plant, Europe's largest nuclear power station. The head of the U.N.'s atomic energy agency spent months trying to negotiate the establishment of a safety perimeter to protect the facility as nearby areas came under repeated shelling, but he has been unsuccessful.

The International Atomic Energy Agency noted Thursday that "the military situation has become increasingly tense" while a Ukrainian counteroffensive that got underway this month unfolds in Zaporizhzhia province, where the namesake plant is located, and in an adjacent part of Donetsk province.

Although the last of the plant's six reactors was shut down last fall to reduce the risk of a meltdown, experts have warned that a radiation release could still happen if the system that keeps the reactors' cores and spent nuclear fuel cool loses power or water.

During months of fighting, Russia and Ukraine have traded blame over which side was increasing the threat to the plant.

Ukrainian officials have accused Russia of mining the plant's cooling system, already under threat from a dam collapse that drew down water in a reservoir used by the power station.

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