Russian President Vladimir Putin said he signed a decree on partial mobilization beginning Wednesday.
Putin said in a televised address on Wednesday that his aim was to "liberate" Ukraine's eastern Donbas region.
He said he was defending Russian territories and people in the region did not want to return to what he called the "yoke" of Ukraine.
The Russian leader stressed he was "taking about partial mobilization, that is, only citizens who are currently in the reserve will be subject to conscription, and above all, those who served in the armed forces have a certain military specialty and relevant experience."
Russian Defense Minister Sergei Shoigu said 300,000 reservists would be called up for partial mobilization.
Putin's address comes a day after Russian-controlled regions in eastern and southern Ukraine announced "referendums" on joining Russia.
The so-called referendums, to be held between September 23 and 27, in Luhansk, Donetsk, Kherson and Zaporizhzhia provinces, represents around 15% of Ukrainian territory, or an area about the size of Hungary.
The "referendums" are similar to what unfurled after 2014, when Russia annexed the Crimean peninsula from Ukraine after a similar vote.
The results are not recognized by the international community, with Washington, Berlin and Paris having denounced latest "referendum" plans as well.
Putin says he is defending Russia
Putin accused the West of "nuclear blackmail," noting "statements of some high-ranking representatives of the leading NATO states about the possibility of using nuclear weapons of mass destruction against Russia."
He said that those who made such statements should know that "our country also has various means of destruction" and "when the territorial integrity of our country is threatened, to protect Russia and our people, we will certainly use all the means at our disposal."
"This is not a bluff," Putin added. He said the West aimed to "weaken, divide and ultimately destroy our country."
Germany, UK, US denounce Putin's lies, vow support for Ukraine
German Vice Chancellor Robert Habeck said Germany was deciding its response to Putin's call for partial mobilization.
Putin's speech was "another bad and wrong step from Russia, which of course we will discuss and consult on politically regarding how to respond," Habeck said.
Gillian Keegan, a minister at the British Foreign Office, told Sky News that Putin's speech was a worrying escalation and that the threats he made should be taken seriously.
"The overall address clearly was more of Putin's lies, it was a rewriting of history," Keegan told the TV channel.
The US Ambassador to Ukraine, Bridget Brink, tweeted that "sham referenda and mobilisation are signs of weakness, of Russian failure"
She added the US would never recognize Russia's claim "to purportedly annexed territory" and the US continues to "stand with Ukraine for as long as it takes."
