After Russian President Vladimir Putin declared war in a pre-dawn televised address, explosions and gunfire were heard throughout the day in Kyiv, a city of three million people.
The assault brought a calamitous end to weeks of fruitless diplomatic efforts by Western leaders to avert war.
"This is a premeditated attack," US President Joe Biden said at the White House as he unveiled harsh new sanctions, co-ordinated with allies, against Russian banks, oligarchs and state companies.
"Putin is the aggressor. Putin chose this war. And now he and his country will bear the consequences," he said.
In his address, Putin said he had ordered "a special military operation" to protect people, including Russian citizens, subjected to "genocide" in Ukraine - an accusation the West calls baseless propaganda.
"And for this we will strive for the demilitarisation and denazification of Ukraine," Putin said.
By nightfall on Thursday, a picture was emerging of fierce fighting across multiple fronts.
The Ukrainian presidential office said Russian forces had captured the Chernobyl former nuclear power plant, north of the capital. There was also fighting at Hostomel airport, just outside Kyiv, where paratroopers had landed. A Ukrainian official later said the airfield had been recaptured.
Heavy exchanges were also reported in the regions of Sumy and Kharkiv in the northeast and Kherson in the south.
The highway heading west out of Kyiv was choked with traffic as residents fled.
The UN refugee agency said an estimated 100,000 Ukrainians had left their homes. Thousands were crossing into Romania, Moldova, Poland and Hungary.
 Ukrainian Health Minister Oleh Lyashko said 57 people had been killed and 169 wounded.
The day began with missiles raining down on targets across Ukraine and reports of troops and armour pouring across the borders from Russia and Belarus to the north and east.
President Volodymyr Zelenskiy called on Ukrainians to defend their country and said arms would be given to anyone prepared to fight.
"What we have heard today are not just missile blasts, fighting and the rumble of aircraft. This is the sound of a new Iron Curtain, which has come down and is closing Russia off from the civilised world," Zelenskiy said.
Putin, after referring to Russia's powerful nuclear arsenal, warned: "Whoever tries to hinder us ... should know that Russia's response will be immediate. And it will lead you to such consequences that you have never encountered in your history."
Foreign Minister Jean-Yves Le Drian said the threat as indeed understood as a use of nuclear weapons, adding Putin should also understand that NATO was a nuclear alliance.
Biden has ruled out sending US troops to defend Ukraine, but Washington has reinforced its NATO allies in the region with extra troops and planes.
After consulting counterparts from the G7, Biden announced measures to impede Russia's ability to do business in the world's major currencies, along with sanctions against banks and state-owned enterprises.
Britain also targeted banks, along with members of Putin's closest circle and super-rich Russians who enjoy lavish London lifestyles.
European Union leaders said its measures would include freezing Russian assets, halting banks' access to European financial markets and hitting "Kremlin interests".
Canada and Switzerland were among a number of other countries to announce new measures targeting Russia.
Putin said he did not plan a military occupation, only to disarm Ukraine and purge it of nationalists, but his endgame remains unclear.
A democratic nation of 44 million people, Ukraine voted for independence at the fall of the Soviet Union and recently stepped up efforts to join NATO and the EU, aspirations that infuriate Moscow.
Putin, who denied for months he was planning an invasion, has called Ukraine an artificial construct, which Ukrainians see as an attempt to erase their more than 1000-year history.
There was also dissent in Russia. Police detained more than 1600 anti-war protesters in 53 cities and authorities threatened to block media reports carrying "false information".
