Russian President Vladimir Putin says 300,000 reservists will now be called up to fight in Ukraine.
In a televised address to the nation, he said this was a necessary step to ensure Russian territorial integrity, and accused the West of wanting to see Russia weakened and destroyed.
After explaining the mobilisation, Putin issued a threat.
“If the territorial integrity of our country is threatened, to defend Russia and our people, we will use all means we have. This is not a bluff.
"The territorial integrity of our motherland, our independence and freedom will be secured, I repeat, with all the means we have.
"Those who try to blackmail us with nuclear weapons should know that the prevailing winds can turn in their direction.”
Meanwhile, Western nations have condemned Moscow's plans to hold urgent so-called referendums in parts of Ukraine that are currently under Russian control.
The US, Germany and France have said they would never recognise the results of such "sham" ballots.
US President Joe Biden and Ukraine's President Zelensky will address world leaders later at the United Nations General Assembly.
Credit: The BBC