Big explosions were heard before dawn in Kyiv, Kharkiv and Odesa as world leaders decried the start of an invasion that could cause massive casualties, topple Ukraine's democratically elected government and threaten the post-Cold War balance on the continent.
Ukrainians started fleeing some cities, and the Russian military claimed to have incapacitated all of Ukraine's air defences and air bases within hours.
President Volodymyr Zelenskyy declared martial law, saying Russia has targeted Ukraine's military infrastructure. Ukrainians who had long braced for the prospect of an assault, while never knowing precisely when it would come, were urged to stay home and not to panic even as the country's border guard agency reported an artillery barrage by Russian troops from neighbouring Belarus.
US President Joe Biden pledged new sanctions to punish Russia for the aggression that the international community had expected for weeks but could not prevent through diplomacy.
Mr Putin justified it all in a televised address, asserting that the attack was needed to protect civilians in eastern Ukraine - a false claim the US had predicted he would make as a pretext for an invasion.
He accused the US and its allies of ignoring Russia's demands to prevent Ukraine from joining Nato and for security guarantees. He also claimed that Russia does not intend to occupy Ukraine but will move to "demilitarise" it and bring those who committed crimes to justice.
Mr Biden in a written statement condemned the "unprovoked and unjustified attack", and he promised that the US and its allies would "hold Russia accountable".
The president said he planned to speak to Americans on Thursday after a meeting of the Group of Seven leaders. More sanctions against Russia were expected to be announced on Thursday.
Ukraine's Foreign Minister Dmytro Kuleba described the assault as a "full-scale invasion" and said Ukraine will "defend itself and will win. The world can and must stop Putin. The time to act is now."
In the capital, Kyiv Mayor Vitaly Klitschko advised residents to stay home unless they are involved in critical work and urged them to prepare go-bags with necessities and documents if they need to evacuate. An Associated Press photographer in Mariupol reported hearing explosions and seeing dozens of people with suitcases heading for their cars to leave the city.
"We are facing a war and horror. What could be worse?" 64-year-old Liudmila Gireyeva said in Kyiv. She planned to head to the western city of Lviv and then to try to move to Poland to join her daughter.
The Russian claims about knocking out Ukrainian air defences and Ukrainian claims to have shot down several Russian aircraft could not immediately be verified. The Ukrainian air defence system and air force date back to the Soviet era and are dwarfed by Russia's massive air power and its inventory of precision weapons.
The Russian Defence Ministry said it was not targeting cities, but using precision weapons and claimed that "there is no threat to civilian population".
Anton Gerashchenko, an adviser to Ukraine's interior minister, said on Facebook that the Russian military had launched missile strikes on Ukrainian military command facilities, air bases and military depots in Kyiv, Kharkiv and Dnipro.
After the initial explosions in Kyiv, people could be heard shouting in the streets. Then a sense of normality returned, with cars circulating and people walking in the streets as a pre-dawn commute appeared to start in relative calm.
The consequences of the conflict and resulting sanctions on Russia could reverberate throughout the world, upending geopolitical dynamics in Europe as well as affecting energy supplies in Europe and jolting global financial markets.
Asian stock markets plunged and oil prices surged as the attack began. Earlier, Wall Street's benchmark S&P 500 index fell 1.8% to an eight-month low after the Kremlin said rebels in eastern Ukraine asked for military assistance.
Anticipating international condemnation and countermeasures, Mr Putin issued a stark warning to other countries not to meddle, saying, "whoever tries to impede us, let alone create threats for our country and its people, must know that the Russian response will be immediate and lead to the consequences you have never seen in history."
Mr Putin urged Ukrainian servicemen to "immediately put down arms and go home".
In a stark reminder of Russia's nuclear power, mR Putin warned that "no one should have any doubts that a direct attack on our country will lead to the destruction and horrible consequences for any potential aggressor".
He emphasiSed that Russia is "one of the most potent nuclear powers and also has a certain edge in a range of state-of-the-art weapons".
Though the US on Tuesday announced the repositioning of forces around the Baltics, Mr Biden has said he will not send in troops to fight Russia.
Mr Putin announced the military operation after the Kremlin said rebels in eastern Ukraine asked Russia for military assistance to help fend off Ukrainian "aggression", an announcement that the White House said was a "false flag" operation by Moscow to offer up a pretext for an invasion.
Mr Putin's announcement came just hours after the Ukrainian president rejected Moscow's claims that his country poses a threat to Russia and made a passionate, last-minute plea for peace.
"The people of Ukraine and the government of Ukraine want peace," Mr Zelensky said in an emotional overnight address, speaking in Russian in a direct appeal to Russian citizens. "But if we come under attack, if we face an attempt to take away our country, our freedom, our lives and lives of our children, we will defend ourselves. When you attack us, you will see our faces, not our backs."
Mr Zelensky said he asked to arrange a call with Mr Putin late on Wednesday, but the Kremlin did not respond.
In an apparent reference to Mr Putin's move to authorise the deployment of the Russian military to "maintain peace" in eastern Ukraine, Mr Zelensky warned that "this step could mark the start of a big war on the European continent".
"Any provocation, any spark could trigger a blaze that will destroy everything," he said.
He challenged the Russian propaganda claims, saying that "you are told that this blaze will bring freedom to the people of Ukraine, but the Ukrainian people are free".
At an emergency meeting of the UN Security Council called by Ukraine because of the imminent threat of a Russian invasion, members still unaware of Mr Putin's announcement appealed to him to stop an attack.
UN secretary-general Antonio Guterres opened the meeting, just before the announcement, telling Mr Putin: "Stop your troops from attacking Ukraine. Give peace a chance. Too many people have already died."
Nato secretary-general Jen Stoltenberg issued a statement condemning "Russia's reckless and unprovoked attack on Ukraine, which puts at risk countless civilian lives. Once again, despite our repeated warnings and tireless efforts to engage in diplomacy, Russia has chosen the path of aggression against a sovereign and independent country".
European Council president Charles Michel and European Commission president Ursula von der Leyen promised to hold the Kremlin accountable.
"In these dark hours, our thoughts are with Ukraine and the innocent women, men and children as they face this unprovoked attack and fear for their lives," they said on Twitter.
Even before Mr Putin's announcement, dozens of nations imposed sanctions on Russia, further squeezing Russian oligarchs and banks out of international markets.
The Russian Foreign Ministry has shrugged off the sanctions, saying that "Russia has proven that, with all the costs of the sanctions, it is able to minimize the damage".