The Russian leader called the invasion a "special military operation" and said Russia did not plan to occupy Ukraine. He also claimed he was helping with the "denazification" of Ukraine, despite Ukraine's leader being Jewish.
He also cautioned against interference from other countries, warning them “if you do you will face consequences greater than any of you have faced in history”.
But the Ukraine government said Putin had “declared war” and Russian forces were entering the country from Belarus in the north and Crimea in the south, as well as Russia to the east.
President Volodymyr Zelensky announced that martial law was now being imposed across all of Ukraine.
"No panic. We're strong. We're ready for anything. We'll defeat everyone, because we are Ukraine," the Ukrainian leader said in a video statement.
US president Joe Biden said Washington and its allies would respond in a united and decisive way to "an unprovoked and unjustified attack by Russian military forces" on Ukraine.
"President Putin has chosen a premeditated war that will bring a catastrophic loss of life and human suffering," Mr Biden said. "The world will hold Russia accountable."
European Commission chief Ursula von der Leyen said the EU stood with Ukraine and that Russia "must withdraw its military".
Boris Johnson said he was "appalled by the horrific events in Ukraine" and that Putin "has chosen a path of bloodshed and destruction by launching this unprovoked attack".
Here's how other politicians, journalists and people reacted to the news:
Putin has just launched a full-scale invasion of Ukraine. Peaceful Ukrainian cities are under strikes. This is a war of aggression. Ukraine will defend itself and will win. The world can and must stop Putin. The time to act is now.
Canada condemns \u2013 in the strongest possible terms \u2013 Russia\u2019s egregious attack on Ukraine. These unprovoked actions are a clear further violation of Ukraine\u2019s sovereignty and territorial integrity, and of Russia\u2019s obligations under international law and the Charter of the UN.
We pray for the people of Ukraine as they defend against Putin\u2019s attempt to rebuild the old Soviet Union. Biden must stand up to Putin and immediately levy severe sanctions against Russia \u2014 starting with removal from the SWIFT banking system. \n\nThe USA stands with Ukraine.
— Sen. Marsha Blackburn (@Sen. Marsha Blackburn)
I stand with Ukraine. They are in the front line, but war in Europe looms, and all our lives, livelihoods, and democratic traditions are at stakehttps://twitter.com/Mariia_Zolkina/status/1496544033874878465\u00a0\u2026
It is clear that Putin never had any interest in peace, security or the self-determination of Ukraine. The entire world should unite in revulsion against him. The people who made excuses and apologies for him should feel utterly ashamed.
This day of shame is tearing families apart. It is causing huge pain to so many. And for nothing. Ukraine threatened no one and Ukrainians are now suffering. It\u2019s a tragedy.https://twitter.com/ragipsoylu/status/1496746417083367424\u00a0\u2026
Overnight developments in Ukraine - however anticipated - are appalling and horrific. The Kremlin must face the severest consequences for this unprovoked aggression. And the world must stand in solidarity with the people of Ukraine \n#StandWithUkraine