NATO chief Jens Stoltenberg strongly condemns what the alliance describes as a careless and unprovoked attack that endangers the lives of countless people.
"Russia has chosen the path of attack towards a sovereign and independent country," he said in a statement.
US President Joe Biden said in a statement that Ukraine had been subjected to an "unprovoked and unjust attack by Russian military forces".
He is expected to speak to the American people later today.
UN: Withdraw your soldiers
UN Secretary-General António Guterres called on Putin not to attack Russia during his speech at the UN Security Council meeting.
Before the meeting was over, Putin had announced that the military operation would begin.
After the meeting, Guterres updated his message:
"President Putin, in the name of humanity, withdraw your troops to Russia," he wrote on Twitter.
Von der Leyen: The thoughts of all the innocent
EU Commission President Ursula von der Leyen condemns the Russian military's attacks on targets in Ukraine.
According to her, the Union will hold Russia "accountable".
Von der Leyen also says that her thoughts are with all the hideous people who now fear for their lives in this "dark moment".
Andersson (S): Russia solely responsible
Sweden's Prime Minister Magdalena Andersson also joins the crowd that condemns the Russian invasion.
"Sweden strongly condemns Russia's ongoing invasion of Ukraine.
Russia's actions are also an attack on the European security order.
It will be met by a united and robust response in solidarity with Ukraine.
Russia is solely responsible for human suffering, "she tweeted via the Prime Minister's official account.
The Prime Minister of the United Kingdom Boris Johnson tweets:
"I am appalled by the horrific events in Ukraine and I have spoken to President Zelensky to discuss the next step.
The Prime Minister of the Netherlands Mark Rutte and the Prime Minister of Canada Justin Trudeau, among others, have also condemned the Russian invasion.