Russia has launched a wide-ranging attack on Ukraine, hitting cities and bases with persistent, prolonged air strikes and shelling, as civilians piled into trains and cars to flee.
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Ukraine's government said Russian tanks and troops rolled across the border in a "full-scale war" that could rewrite the geopolitical order and whose fallout is reverberating around the world.
“Russia is not only attacking Ukraine, but the rules of normal life in the modern world," Ukrainian presidential adviser Mykhailo Podolyak said.
In a video address, Ukraine's President Volodymyr Zelenskyy said 137 people have been killed across Ukraine, including soldiers and civilians.
He called them "heroes", and said hundreds more have been wounded.
Mr Zelenskyy said despite Russia's claim that it was only attacking military targets, civilian sites have also been struck.
"They're killing people and turning peaceful cities into military targets," he said.
"It's foul and will never be forgiven."
In unleashing Moscow's most-aggressive action since the Soviet invasion of Afghanistan, President Vladimir Putin deflected global condemnation and cascading new sanctions and threatened any foreign country attempting to interfere with "consequences you have never seen".
From Thursday morning and throughout the afternoon, sirens rang out in Ukraine’s capital of Kyiv, with large explosions heard there and in other cities.
Queues of people waited to withdraw money and buy supplies of food and water in Kyiv.
Other citizens flooded onto public transport and took to busy roads to make their escape as the government said the former Soviet republic was seeing a long-anticipated invasion from the east, north and south.
Traffic going out of major cities towards the Polish border was jammed and that nation had said it was preparing for an influx of refugees.
Cars stretched back for dozens of kilometres on the highway to the western city of Lviv, witnesses said.
"We're afraid of bombardments," said Oxana, stuck in her car with her three-year-old daughter on the back seat.
Ukrainians in disbelief that war is upon them
Even after weeks of warnings by Ukrainian and Western politicians that a Russian attack was imminent, some people were caught off guard in the city of around three million people.
"I didn't expect this. Until this morning I believed nothing would happen," said Nikita, a 34-year-old marketing specialist, as he waited in a long line at a supermarket with bottles of water piled high in his shopping trolley.
"I was woken up. I'm an adult, healthy man. I packed, bought food and will stay at home with my family."
The instant a missile fragment pierced the ceiling of Mikhail Shcherbakov's apartment in Kharkiv was when he found that war, after weeks of warnings, had hit home.
"I heard noise and woke up. I realised it sounded like artillery," Mr Shcherbakov said from the country's north-east.
He jumped from the couch and ran to wake his mother, and something exploded behind him.
The missile left a nearby computer and teacup shrouded in dust, instant artefacts of Europe's latest war.
Emergency services in Kharkiv said a child was killed during the shelling.
Government officials said fighting there was happening 4 to 5 kilometres north of the city.
As missiles struck across the country, some panicked immediately. Others clung to routine, with irritation.
Many seemed unsure of how to react.
Kyiv's main street, Khreshchatyk, rippled with anxiety as people checked their phones. Some walked their dogs or waved at friends.
"I'm not scared at the moment. Maybe I'll be scared later," resident Maxim Prudskoi said.
Kyiv Mayor Vitaly Klitschko called on the city's three million people to stay indoors unless they worked in critical sectors and said everyone should prepare bags with necessities such as medicines and documents.
For weeks, Mr Zelenskyy had tried to moderate expectations of aggression by Russia, even as warnings by the United States became more urgent.
Mr Zelenskyy argued that panic would lead to societal destabilisation that could be as much of a tactical advantage for Russia as the estimated 150,000 troops who had been amassed on Ukraine’s borders.
On Thursday, as the President imposed martial law, Ukrainians realised — with a jolt — that everything might change.
“I feel panic, scared and excited. I don’t know who I should ask for help,” said Kyiv resident Elizaveta Melnik. "We didn’t believe this situation would come.”
Western countries and Ukraine's neighbours are preparing for the likelihood of hundreds of thousands of refugees as a result of the invasion.
The head of the UN refugee agency is warning of "devastating consequences" of Russia's military action in Ukraine and calling on neighbouring countries to keep their borders open for people fleeing the fighting.
Filippo Grandi, the UN High Commissioner for Refugees, pointed to "reports of casualties and people starting to flee their homes to seek safety" without elaborating.
In a statement, he said that UNHCR had stepped up its operations and capacity in Ukraine and its neighbouring countries, without providing further details.
ABC/wires
