Mayor of Kyiv Vitali Klitschko has told ITV he has "no other choice but take up arms" after Vladimir Putin launched an invasion of Ukraine, adding that he "will be fighting" Russia if war is announced.
The former heavyweight champion boxer said the people of Kyiv have weapons and have become "soldiers ready to defend our city", which he believes is under threat of a Russian takeover.
Asked if he believes it is possible to stave off the invasion, Klitschko said: "I believe. I believe in Ukraine, I believe in my country and I believe in my people."
He said President Putin has "totally" lost all sense of reality after starting a "bloody war".
"We don't have exact number but we know many, many examples" of Ukrainians dying amid Russia's attack.
Asked if he would join civilians and soldiers in fighting Russian forces, Klitschko said "I don't have another choice. I have to do that," adding: "I will be fighting."
He thanked the West for supporting Ukraine after Boris Johnson announced a "massive" package of sanctions "designed in time to hobble the Russian economy".
In an address to the nation, Mr Johnson said: "Our mission is clear diplomatically, politically, eventually military, this hideous and barbaric venture of Vladimir Putin must end in failure."
He added that the world could not allow the freedom of Ukraine to be "snuffed out" and "we cannot and will not just look away".
The Russian invasion was launched overnight, with President Putin telling the West any interference would lead to "consequences you have never seen."
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Ahead of launching his invasion, the strong-arm leader warned Ukrainians to "lay down your arms immediately and go home".
"All Ukrainian soldiers who comply with this requirement can freely leave the area of military action and return to their families," Putin said.
Watch Boris Johnson's address to the nation in full
Klitschko earlier released a video alongside his brother and fellow ex-boxer Wladimir after the invasion began, pleading for help from the west and condemning a "senseless war that is not going to have any winners, but losers".
Speaking in English, Wladimir said: “I’m calling to all international partners to observe this tragedy that is happening nowadays in Ukraine.
“And this senseless war that is not going to have any winners, but losers.
“I just want to tell you we must stay united against this aggression, against Russian aggression.
“Don’t let it continue happening in Ukraine, don’t let it happen in Europe and eventually in the world.
“United we are strong, support Ukraine, thank you.”