Russian President Vladimir Putin on Saturday vowed harsh punishment for the organisers of an armed rebellion spearheaded by mercenary chief Yevgeny Prigozhin, who led his troops out of Ukraine and advanced toward Moscow.

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

As Prigozhin's forces rolled toward the capital, military trucks and armored vehicles were seen in several parts of Moscow. On its southern edge, troops erected checkpoints, arranged sandbags and put up machine guns.

Authorities declared a “counterterrorist regime” in the capital and its surrounding region, enhancing security and restricting some movement.

Moscow Mayor Sergei Sobyanin urged residents not to drive and said key city services were on high readiness. He declared Monday to be a nonworking day for most residents except publice servants and some industrial enterprises.

Crews also dug up parts of highways in an apparent bid to slow the march of the Wagner mercenary army. Access to Red Square was closed, two major museums were evacuated and a park was shut.

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

Wagner troops and equipment also were in Lipetsk province, about 360 kilometers (225 miles) south of Moscow, where authorities “are taking all necessary measures to ensure the safety of the population," said regional Gov Igor Artamonov, via Telegram. He did not elaborate.

The dramatic developments came exactly 16 months after Russia launched its full-scale invasion of Ukraine, Europe's largest conflict since World War II, that has killed tens of thousands, displaced millions and reduced cities to rubble.

Ukrainians hoped the Russian infighting would create opportunities for its army to take back territory seized by Russian forces.

Ukrainian President Volodymyr Zelenskyy said Moscow is suffering “full-scale weakness” and that Kyiv was protecting Europe from “the spread of Russian evil and chaos.”

In his speech, 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.”

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

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 army has been fighting alongside regular 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 said he had 25,000 troops under his command and urged the army not to offer resistance.

He posted video of himself at the military headquarters in Rostov-on-Don and claimed 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.

“We didn't kill a single person on our way,” Prigozhin said in one of his several messages posted as the day went on, adding that his forces seized the military headquarters “without a single gunshot.” His claims could not be independently verified. The Russian authorities haven't reported any casualties so far, either.

The rebellion comes as Russia is “fighting the toughest battle for its future,” Putin said, with the West piling sanctions on Moscow and arming Ukraine.

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

A Muscovite who gave only his first name of Khachik called the situation “scary.” Another man who didn't want to be identified at all denounced Prigozhin's move as a betrayal and said he supports the Defense Ministry.

State-controlled TV networks led their newscasts with Putin's statement and reported the tense situation in Rostov-on-Don. Some showed social media videos of residents denouncing Wagner troops.

Broadcasters also carried statements from top officials and lawmakers voicing support for Putin and condemning Prigozhin.

In announcing the rebellion, 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 his 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.

The 62-year-old Prigozhin, a former convict, has long ties to the Russian leader and won lucrative Kremlin catering contracts that earned him the nickname “Putin's chef.”

He gained attention in the U.S. when he and a dozen other Russian nationals were charged with operating a covert social media campaign aimed at fomenting discord ahead of Donald Trump's 2016 presidential election victory. He formed the Wagner mercenary group, which sent military contractors to Libya, Syria, several African countries and eventually Ukraine.

After Putin's address, in which he called for unity, officials sought to reiterate their allegiance to the Kremlin and urged Prigozhin to back down.

Vyacheslav Volodin, speaker of the lower house of parliament, said lawmakers “stand for the consolidation of forces? and support Putin.

Foreign Ministry spokeswoman Maria Zakharova echoed that, saying in a Telegram post that “we have one commander in chief. Not two, not three. One.?

Ramzan Kadyrov, the strongman leader of the Chechnya region who used to side with Prigozhin in his criticism of the military, also expressed his full support of Putin's “every word.”

“The mutiny needs to be suppressed," Kadyrov said.

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

Wagner forces have played a crucial role, capturing the eastern city of Bakhmut, an area where the bloodiest and longest battles have taken place. But Prigozhin has increasingly criticized the military brass, accusing it of incompetence and of starving his troops of munitions.

Zelenskyy noted the rebellion in his Telegram channel and said “anyone who chooses the path of evil destroys himself.”

“For a long time, Russia used propaganda to mask its weakness and the stupidity of its government. And now there is so much chaos that no lie can hide it,” he said.

Prigozhin's actions could have significant implications for the war. Orysia Lutsevych, the head of the Ukraine Forum at the Chatham House think tank in London, said the infighting will create confusion and potential division among Russian military forces.

“Russian troops in Ukraine may well now be operating in a vacuum, without clear military instructions, and doubts about whom to obey and follow,? Lutsevych said. “This creates a unique and unprecedented military opportunity for the Ukrainian army.”

Ukrainian soldier Andrii Kvasnytsia, attending a funeral for a comrade, said Prigozhin's intentions toward Ukraine might be worse than Putin's, but that the infighting would still benefit the country.

Prigozhin, whose feud with the Defense Ministry dates back years, had refused to comply with a requirement that his forces sign contracts with the ministry before July 1. He said Friday he was ready for a compromise but “they have treacherously cheated us.”

In Washington, the Institute for the Study of War said “the violent overthrow of Putin loyalists like Shoigu and Gerasimov would cause irreparable damage to the stability of Putin's perceived hold on power.”

Western countries monitored developments closely. US Secretary of State Antony Blinken spoke with his counterparts in the other G7 countries and the European Union's foreign affairs representative, his spokesman said, adding that Blinken “reiterated that support by the United States for Ukraine will not change.”

Latvia and Estonia, two NATO countries that border Russia, said they were increasing security at their borders.

The Kremlin said Putin spoke by phone with the leaders of Turkey, Belarus, Kazakhstan and Uzbekistan about the events.

Although there was speculation that Putin had left Moscow, his spokesman Dmitry Peskov denied it.