After using airstrikes on cities and military bases, Russian military units moved swiftly to take on Ukraine's seat of government and its largest city in what US officials suspect is a brazen attempt by Russian President Vladimir Putin to dismantle the government and replace it with his own regime.
Ukrainian leaders pleaded for help as civilians piled into trains and cars to flee, and thousands took shelter in the Kyiv's subways.
Ukrainian forces braced for more attacks after enduring a Russian barrage of land- and sea-based missiles.
Ukraine officials said they had lost control of the decommissioned Chernobyl nuclear power plant, scene of the world's worst nuclear disaster.
Alyona Shevtsova, adviser to the commander of Ukraine's ground forces, wrote on Facebook that staff members at the Chernobyl plant had been "taken hostage." The White House said it was "outraged" by reports of the detentions.
In unleashing the largest ground war in Europe since World War II, Putin ignored global condemnation and cascading new sanctions. With a chilling reference to his country's nuclear arsenal, he threatened any country trying to interfere with "consequences you have never seen," as a once-hoped for diplomatic resolution now appeared impossible.
"Russia has embarked on a path of evil, but Ukraine is defending itself and won't give up its freedom," Zelenskyy tweeted. He pleaded for more severe sanctions than those imposed so far by Western allies and ordered a full military mobilisation that would last 90 days.
Zelenskyy said in a video address that 137 "heroes," including 10 military officers, had been killed and 316 people wounded. The dead included all border guards on the Zmiinyi Island in the Odesa region, which was taken over by Russians.
He appealed to global leaders, saying that "if you don't help us now, if you fail to offer a powerful assistance to Ukraine, tomorrow the war will knock on your door."
US President Joe Biden announced a series of new sanctions against Russia, saying Putin "chose this war".
The US sanctions will target Russian banks, oligarchs, state-controlled companies and high-tech sectors, Biden said, but they were designed not to disrupt global energy markets. Russian oil and natural gas exports are vital energy sources for Europe.
Biden also said the US was deploying additional forces to Germany to bolster NATO.
The chief of the NATO alliance, Jens Stoltenberg, said Russia's "brutal act of war" shattered peace in Europe, joining a chorus of world leaders decrying the attack.
British Prime Minister Boris Johnson said he aimed to cut off Russia from the UK's financial markets as he announced sanctions, freezing the assets of all large Russian banks and planning to bar Russian companies and the Kremlin from raising money on British markets.
"Now we see him for what he is — a bloodstained aggressor who believes in imperial conquest," Johnson said of Putin.
Zelenskyy urged the US and West to go further and cut the Russians from the SWIFT system, a key financial network that connects thousands of banks around the world. The White House has been reluctant to immediately cut Russia from SWIFT, worried it could cause enormous economic problems in Europe and elsewhere in the West.
Russia and Ukraine made competing claims about damage they had inflicted. Russia's Defence Ministry said it had destroyed scores of Ukrainian air bases, military facilities and drones. It confirmed the loss of one of its Su-25 attack jets, blaming "pilot error," and said an An-26 transport plane had crashed because of technical failure, killing the entire crew. It did not say how many were aboard.
