President Vladimir Putin vowed Saturday to punish the organizers of an armed rebellion in Russia after mercenary chief Yevgeny Prigozhin led his troops out of Ukraine and into a key southern city.

Putin denounced the uprising as “a stab in the back." It was the biggest threat to his leadership in over two decades in power.

The private army led by Prigozhin appeared to control the military headquarters in Rostov-on-Don, a city 660 miles (over 1,000 kilometers) south of Moscow that runs Russian offensive operations in Ukraine, Britain’s Ministry of Defense said.

As the fast-moving events unfolded in Russia, Ukrainian President Volodymyr Zelensky said Moscow is suffering “full-scale weakness” and that Kyiv was protecting Europe from “the spread of Russian evil and chaos.”

In his address, Putin called the actions by Prigozhin, whom he did not mention by name, a “betrayal” and “treason.”

“All those who prepared the rebellion will suffer inevitable punishment,” Putin said. “The armed forces and other government agencies have received the necessary orders.”

Prigozhin said his fighters would not surrender, as “we do not want the country to live on in corruption, deceit and bureaucracy.”

“Regarding the betrayal of the motherland, the president was deeply mistaken. We are patriots of our homeland," he said in an audio message on his Telegram channel.

Prigozhin’s private military contractor, known as Wagner, has been fighting alongside Russian troops in Ukraine. His goals weren't immediately clear, but the rebellion marks an escalation in his struggle with Russian military leaders, whom he accused of botching the war in Ukraine and hobbling his forces in the field.

“This is not a military coup, but a march of justice,” Prigozhin said.

Prigozhin confirmed Saturday he and his troops reached Rostov-on-Don after crossing the border from Ukraine.

He posted a video of himself at the military headquarters in Rostov and claimed that his forces had taken control of the airfield and other military facilities in the city. Other videos on social media showed military vehicles, including tanks, on the streets.

Prigozhin said his forces faced no resistance from young conscripts as they crossed into Russia, saying his troops “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.”

The rebellion comes at a time when Russia is “fighting the toughest battle for its future,” Putin said, as Western governments heap sanctions on Moscow and arm Ukraine.

“The entire military, economic and information machine of the West is waged against us,” Putin said.

Russia’s security services, including the Federal Security Service, or FSB, called for Prigozhin’s arrest after he declared an armed rebellion late Friday.

In a sign of how seriously the Kremlin took the threat, authorities declared a “counterterrorist regime” in Moscow and its surroundings, allowing restricted freedoms and enhancing security in the capital.

It was not immediately clear how Prigozhin was able to enter the southern Russian city or how many troops he had with him.

Prigozhin said he wanted to punish Defense Minister Sergei Shoigu after he accused Russian government forces of attacking Wagner field camps in Ukraine with rockets, helicopter gunships and artillery. He claimed that “a huge number of our comrades got killed.”

Prigozhin said Wagner’s forces shot down a Russian military helicopter that fired on a civilian convoy, but there was no independent confirmation of that.

He alleged that Gen. Valery Gerasimov, chief of the General Staff, ordered the attacks following a meeting with Shoigu, where they decided to destroy Wagner.

The Defense Ministry denied attacking the Wagner camps.

Prigozhin said he had 25,000 troops under his command and urged the army not to offer resistance.