Russian President Vladimir Putin launched an invasion of Ukraine on Thursday, forcing residents to flee for their lives and leaving at least 40 Ukrainian soldiers and 10 civilians dead.
Russian airstrikes hit military facilities across the country and ground forces moved in from the north, south and east, triggering condemnation from Western leaders and warnings of massive sanctions.
Weeks of intense diplomacy to avert war failed to deter Putin, who had massed more than 150,000 troops along the borders of Ukraine.
"I have decided to proceed with a special military operation," Putin said in a television announcement in the early hours of Thursday.
Shortly afterwards, the first bombardments were heard in Ukraine's capital, Kyiv, and several other citiess.
"Putin has just launched a full-scale invasion of Ukraine. Peaceful Ukrainian cities are under strikes," Ukrainian Foreign Minister Dmytro Kuleba tweeted.
President Volodymyr Zelensky declared martial law and said Russia was attacking his country's "military infrastructure" but urged citizens not to panic and vowed victory.
The military said it had received orders from Zelensky to "inflict maximum losses against the aggressor."
It said its forces had killed "around 50 Russian occupiers" while repulsing an attack on a town on the frontline with Moscow-backed rebels, a toll that could not be immediately confirmed by AFP.
'Sounds of bombing'
Kyiv's main international airport was hit in the first bombing of the city since World War II and air raid sirens sounded over the capital at the break of dawn.
"I woke up because of the sounds of bombing. I packed a bag and tried to escape," said Maria Kashkoska, as she sheltered inside the Kyiv metro station.
In the eastern Ukrainian town of Chuguiv, a son wept over the body of his father among the wreckage of a missile strike in a residential district.
"I told him to leave," the man sobbed repeatedly, next to the twisted ruins of a car.
'The time to act is now'
Kuleba said the worst-case scenario was playing out.
"This is a war of aggression. Ukraine will defend itself and will win. The world can and must stop Putin. The time to act is now," he said.
Within a few hours of Putin's speech, Russia's Defence Ministry said it had neutralised Ukrainian military airbases and its air defence systems.
Ukraine said Russian tanks and heavy equipment crossed the border in several northern regions, in the east as well as from the Kremlin-annexed peninsula of Crimea in the south.
The Russian Army said Moscow-backed separatists in the east had advanced by up to three kilometres (1.8 miles) into territory previously under government control.
The fighting roiled the global financial markets, with stocks plunging and oil prices soaring past US$100.
The Russian ruble fell nine percent against the dollar after the attack and the Moscow Stock Exchange was down more than 25 percent.
'Unprovoked and unjustified'
In his televised address, Putin justified the operation by claiming the government was overseeing a "genocide" in the east of the country.
The Kremlin had earlier said the leaders of two separatist territories in eastern Ukraine had asked Moscow for military help against Kyiv.
Putin recognised the independence of the self-proclaimed Donetsk and Lugansk republics on Monday.
US President Joe Biden spoke with Zelensky after the Russian operation began to vow US "support" and "assistance."
Biden condemned the "unprovoked and unjustified attack by Russian military forces," and vowed Russia would be held accountable.
"President Putin has chosen a premeditated war that will bring a catastrophic loss of life and human suffering," he said in a statement.
Biden was due to join a virtual, closed-door meeting of G7 leaders – Britain, Canada, France, Germany, Italy, Japan and the United States – on Thursday, likely to result in more sanctions against Russia.
In Brussels, EU foreign policy chief Josep Borrell said Russia faced "unprecedented isolation" and would be hit with the "harshest sanctions" the EU has ever imposed.
The Russian invasion also rattled other countries in eastern Europe once dominated by Moscow.
NATO member Poland said it was invoking Article 4 of the NATO Treaty, calling for urgent consultations among leaders of the Western military alliance.
Lithuania joined Poland's call and said it would impose a national state of emergency.
Drop NATO ambitions
Ukraine has around 200,000 military personnel, and could boost that with up to 250,000 reservists.
Moscow's total forces are much larger – around a million active-duty personnel – and have been modernised and re-armed in recent years.
But Ukraine has received advanced anti-tank weapons and some drones from NATO members. More have been promised as the allies try to deter a Russian attack or at least make it costly.
Russia has long demanded that Ukraine be forbidden from ever joining the NATO alliance and that US troops pull out from Eastern Europe.
Putin this week set out a number of stringent conditions if the West wanted to de-escalate the crisis, saying Ukraine should drop its NATO ambition and become neutral.
"Putin's aim is to end the existence of Ukraine," said Tatyana Stanovaya, founder of the political consultancy R.Politik Center and a non-resident scholar at the Carnegie Moscow Center.
"It is possible that the east of Ukraine will come under Russian control," she said, adding: "I cannot see anything that would stop Russia now."
by Dave Clark in Kyiv with Thibaut Marchand in Chuguiv, AFP