Ukrainian forces battled Russian invaders on three sides after Moscow mounted an assault by land, sea and air in the biggest attack on a European state since the Second World War.
After Russian President Vladimir Putin declared war in a pre-dawn televised address, explosions and gunfire were heard throughout the morning in Kyiv, a city of three million people.
Missiles rained down on Ukrainian targets and authorities reported columns of troops pouring across Ukraine's borders from Russia and Belarus to the north and east, and landing on the southern coasts from the Black Sea and Azov Sea.
Across Ukraine, at least 137 "heroes" were killed after the first day of fighting, President Volodymyr Zelenskyy said, calling up conscripts and reservists nationwide to fight in a general mobilisation.
The assault brought a calamitous end to weeks of fruitless diplomatic efforts by Western leaders to avert war.
After a day of fighting, Mr Putin told business people in Moscow he had no choice but to act, while Western leaders condemned the Russian leader and promised sweeping economic sanctions.
"This hideous and barbarous venture of Vladimir Putin must end in failure," British Prime Minister Boris Johnson told parliament, announcing measures targeting banks, members of Mr Putin's closest circle and super-rich Russians who enjoy high-rolling London lifestyles.
President Volodymyr Zelenskyy 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," Mr Zelenskyy said.
As night fell, the picture of what was happening on the ground was sketchy.
Russia's defence ministry said it had destroyed 83 land-based Ukrainian targets and had achieved all its goals, according to Interfax news agency.
An adviser to the Ukrainian presidential office said Russian forces had captured the Chernobyl nuclear power plant, just 90 kilometres north of the capital, and Hostomel airport in the Kyiv region, where paratroopers had earlier been landed.
Fierce fighting was taking place in the regions of Sumy and Kharkiv in the northeast and Kherson and Odessa in the south.
The highway heading west out of Kyiv was choked with traffic across five lanes as residents fled.
Russian troops entered Ukraine while the country's President Volodymyr Zelensky addressed the nation to announce the imposition of martial law. Source: EPA / SERGEY DOLZHENKO/EPA
In his address, Mr 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.
US President Joe Biden called the Russian action an "unprovoked and unjustified attack" and EU foreign affairs chief Josep Borrell said: "These are among the darkest hours of Europe since the Second World War."
A resident of Kharkiv, Ukraine's second-biggest city and close to the Russian border, said windows in apartment blocks were shaking from constant blasts.
Ukrainians take shelter in a metro station after air raid sirens alarm in Kyiv. Source: EPA / STRINGER/EPA
Blasts could be heard in the southeasterly port of Mariupol, near a frontline held by Russian-backed separatists in eastern Ukraine. Civilians in Mariupol packed bags, with one woman saying: "We are going into hiding."
Ukraine's ambassador to the United States said 40 servicemen and dozens of civilians had been reported killed, but the information was not up-to-date.
Authorities in the southwesterly Odessa region said 18 people had been killed in a missile attack. At least six people were killed in Brovary, a town near Kyiv, authorities there said.
Ukraine's military said it had destroyed four Russian tanks near Kharkiv, killed 50 troops near a town in Luhansk region, and downed six Russian warplanes in the east.
Russia denied that any of its aircraft or armoured vehicles had been destroyed. Russian-backed separatists said they had downed two Ukrainian planes.
Even with a full-blown invasion underway, Mr Putin's ultimate aim is obscure. He said he did not plan a military occupation, only to disarm Ukraine and purge it of nationalists.
The outright annexation of such a vast, hostile country could be beyond even Russia's military capabilities.
A senior US defence official said Washington believed the invasion was intended to "decapitate" Mr Zelenskyy's government.
But it is hard to see Ukrainians accepting any new leadership installed by Moscow.
"I think we must fight all those who invade our country so strongly," said one man stuck in traffic trying to leave Kyiv. "I would hang every single one of them from bridges."
Mr 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.
Russia is one of the world's biggest energy producers, and both it and Ukraine are among the top exporters of grain.
War and sanctions will disrupt economies around the world already facing a supply crisis as they emerge from the coronavirus pandemic.
A democratic nation of 44 million people, Ukraine is Europe's biggest country by area after Russia itself. It voted for independence at the fall of the Soviet Union and aims to join NATO and the European Union, aspirations that infuriate Moscow.
Ukrainians protest in front of the Russian embassy in Kyiv Source: EPA / SERGEY DOLZHENKO/EPA
Mr Putin, who denied for months that he was planning an invasion, has called Ukraine an artificial construct carved from Russia by its enemies - a characterisation Ukrainians see as an attempt to erase their more than 1,000-year-old history.
While many Ukrainians, particularly in the east, speak Russian as a native language, virtually all identify themselves as Ukrainian.
In Kyiv, queues of people waited to withdraw money and buy supplies of food and water.
Cars stretched for dozens of kilometres on the highway leading west towards Poland, where Western countries have prepared to receive hundreds of thousands of refugees.
