Updated at 10:20 P.M. EDT

KYIV, Ukraine  — The owner of the Wagner private military force is escalating his direct challenge to the Kremlin, calling for an armed rebellion aimed at ousting Russia’s defense minister and claiming some of his soldiers have crossed into Russia from Ukraine. Russian security services reacted immediately by calling for the arrest of Yevgeny Prigozhin.

In a sign of how seriously the Kremlin was taking the threat from the 25,000 soldiers Mr. Prigozhin says he has under his command, security was heightened at Moscow and at 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.

Mr. 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.”

Mr. Prigozhin had earlier posted a series of angry video and audio recordings in which he accused Russia’s Defense minister, Sergei Shoigu, of ordering a rocket strike Friday on Wagner’s field camps in Ukraine, where his troops are fighting on behalf of Russia.

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

[This comes as President Putin is facing growing restiveness among minority regions throughout the republic, as The New York Sun’s James Brooke reports this evening.]

The National Anti-Terrorism Committee, which is part of the Federal Security Services, said the mercenary chief would be investigated on charges of calling for an armed rebellion. The state news agency Tass said President Putin was being kept informed.

Heavy military trucks and armored vehicles were seen at 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 Federal Security Services, people were dancing in the street near the entrance.

Wagner’s 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.

Mr. Prigozhin has frequently criticized Russia’s military brass, accusing it of incompetence and of starving his troops of weapons and ammunition, but his accusations and calls for armed rebellion Friday were more direct challenge.

The Russian Defense Ministry required all military contractors to sign contracts with it before July 1, but Mr. Prigozhin, whose feud with the Defense Ministry dates back years, has refused to comply.

In a statement issued late Friday, he said he was ready to find a compromise with the Defense Ministry, 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.

Mr. Prigozhin claimed that Mr. Shoigu went to the Russian military headquarters in the southern city of Rostov-on-Don personally to direct the strike on Wagner and then “cowardly” fled.

“This scum will be stopped,” he said, in a reference to Mr. 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.”

A top Russian officer, Lieutenant General Vladimir Alexeyev, denounced Mr. 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.”

ZELENSKY WARNS OF RUSSIAN THREAT TO NUCLEAR PLANT

In other developments in the Ukraine war, President Zelensky 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. Mr. Zelensky said that he expected other nations to “give appropriate signals and exert pressure” on Moscow.

“Our principle is simple: The world must know what the occupier is preparing. Everyone who knows must act,” Mr. Zelensky said late Thursday. “The world has enough power to prevent any radiation incidents, let alone a radiation catastrophe.”

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 UN’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.