"This horrific moment in history sends a terrifying message: the message that in this day and age, in year 2022, it's okay to violate the rights of free people and allow autocrats like Putin to take whatever they please."
Panettiere says she wishes she could provide more support to Ukraine.
She said: "I'm praying for my family and friends there and everyone who's fighting. I wish you had more support and I wish I was there fighting with you!"
The actor also encouraged her followers to voice their support for democracy.
She explained: "For now, I ask for those of us who can't be there to stand shoulder to shoulder in solidarity with the people of Ukraine and show your support for #democracy."
Meanwhile, Klitschko recently enlisted in Ukraine's reserve army.
The 45-year-old former heavyweight champion and his brother Vitali Klitschko, who is also a former heavyweight champion, have both pledged to fight for their country.
Wladimir said: "We will defend ourselves with all our might and fight for freedom and democracy. You can also act. Let not fear seize us; let's not remain frozen. Putin shoots at Ukrainian cities, but he aims at our hearts and, more importantly, at our minds."