Russian President Vladimir Putin has called an armed rebellion led by the Wagner mercenary group chief a "betrayal", and has promised to "defend the people" and Russia.
Mr Putin addressed the nation on Saturday morning after mercenary chief Yevgeny Prigozhin and his troops reached the key Russian city of Rostov-on-Don.
The Russian leader said the move by the Wagner group had blocked civilian and military governing bodies in the southern city.
And Mr Putin warned that his country is now facing "the toughest battle for its future".
He said the mutiny amounted to "a deadly threat to our statehood", and vowed "tough actions" in response.
"All those who prepared the rebellion will suffer inevitable punishment. The armed forces and other government agencies have received the necessary orders," Mr Putin said.
He called Mr Prigozhin's actions, without referring to the owner of the Wagner private military company by name, "a betrayal" and "a treason".
Mr Putin urged "those who are being dragged into this crime not to make a fatal and tragic, unique mistake, to make the only right choice - to stop participating in criminal acts".
Mr Prigozhin, the owner of the Wagner private military company, claimed that his forces now control military facilities in Rostov-on-Don.
"We will destroy anyone who stands in our way," Mr Prigozhin said in one of a series of video and audio recordings posted on social media, beginning late on Friday.
"We are moving forward and will go until the end," he added.
Mr Putin condemned the rebellion at a time when Russia was "fighting the toughest battle for its future" with its war in Ukraine.
"The entire military, economic and information machine of the West is waged against us," the Russian leader said.
"This battle, when the fate of our people is being decided, requires the unification of all forces, unity, consolidation and responsibility."
An armed rebellion at a time like this is "a blow to Russia, to its people", Mr Putin added.
"Those who plotted and organised an armed rebellion, who raised arms against his comrades-in-arms, betrayed Russia. And they will answer for it," the Russian President said.
Russia's security services responded to Mr Prigozhin's declaration of an armed rebellion by calling for his arrest.
In a sign of how seriously the Kremlin is taking the threat, security was heightened in Moscow, Rostov-on-Don and other regions.
It is not immediately clear how Mr Prigozhin was able to enter the southern Russian city or how many troops he had with him.
Mr Prigozhin alleged that Wagner field camps in Ukraine were struck by rockets, helicopter gunships and artillery fire on orders from the chief of the general staff, Gen Valery Gerasimov, following a meeting in Rostov with Russian defence minister Sergei Shoigu at which they decided to destroy Wagner.
He also said his forces shot down a Russian military helicopter that fired on a civilian convoy, but there has been no independent confirmation of this.
Mr Prigozhin said he had 25,000 troops under his command and would punish Mr Shoigu in an armed rebellion.
He and urged the army not to offer resistance, declaring: "This is not a military coup, but a march of justice."
While the outcome of the confrontation is still unclear, it appears likely to further hinder Moscow's war effort as Kyiv's forces were probing Russian defences in the initial stages of a counter-offensive.
The dispute, especially if Mr Prigozhin were to prevail, also could have repercussions for Mr Putin and his ability to maintain a united front.
