Protests are expected across Russia later against President Putin's mobilisation of 300-thousand more troops to Ukraine.
The move has caused many men of fighting age to flee Moscow and St Petersburg.
There are also concerns about planned referendums that will allow Russia to illegally claim four occupied regions of Ukraine as their own.
Sky correspondent Sally Lockwood is in Kyiv and says it could mark a turning point for the invasion:
"Russian civilians are now impacted by this war in a way that they haven't been so far. And Vladimir Putin also made his most explicit threat yet when it came to nuclear weapons. Many are calling it desperation. It's certainly escalation, and it certainly feels like a tense moment in this war".
