Russian President Vladimir Putin launched a military operation in Ukraine on Thursday with explosions heard across the country and its foreign minister warning a "full-scale invasion" was underway.
Weeks of intense diplomacy and the imposition of Western sanctions on Russia failed to deter Putin, who had massed between 150,000 and 200,000 troops along the borders of Ukraine.
"I have made the decision of a military operation," Putin said in a surprise television announcement that triggered immediate condemnation from US President Joe Biden and other Western leaders, and sent global financial markets into turmoil.
Shortly after the announcement, explosions were heard in Ukraine's capital, Kyiv, and several other cities, according to AFP correspondents.
Ukrainian border guards reported being under attack along the Russian and Belarusian frontiers.
Ukrainian leader Volodymyr Zelensky said in a Facebook post that Russia was attacking his country's "military infrastructure", but urged citizens not to panic and vowed victory.
His foreign minister said the worst-case scenario was playing out.
"Putin has just launched a full-scale invasion of Ukraine. Peaceful Ukrainian cities are under strikes," Dmytro Kuleba tweeted.
"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."
In his televised address, Putin called on Ukrainian soldiers to lay down their arms, and justified the operation by claiming the government was overseeing a "genocide" in the east of the country.
The Kremlin had earlier said rebel leaders in eastern Ukraine had asked Moscow for military help against Kyiv.
Biden, who had for weeks sought to lead a Western alliance to deter Putin from invading Ukraine, 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 urged world leaders to speak out against Putin's "flagrant aggression".
He also 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.
"Russia alone is responsible for the death and destruction this attack will bring, and the United States and its allies and partners will respond in a united and decisive way.
Biden was due to join a virtual, closed-door meeting of G7 leaders - Britain, Canada, France, Germany, Italy, Japan and the United States - at 9am (1400 GMT) Thursday.
The G7 meeting is likely to result in more sanctions against Russia, which had long claimed it would not invade Ukraine, despite massing troops on the country's borders.
'Aggression'
The excuse for the military operation was given on Wednesday when the Kremlin said the separatist leaders of Donetsk and Lugansk had sent separate letters to Putin, asking him to "help them repel Ukraine's aggression".
The two letters were published by Russian state media and were both dated February 22.
Their appeals came after Putin recognised their independence and signed friendship treaties with them that include defence deals.
The United Nations Security Council had met late Wednesday for its second emergency session in three days over the crisis, with a personal plea there by UN Secretary-General Antonio Guterres coinciding with Putin's announcement.
"President Putin, in the name of humanity, bring your troops back to Russia," Guterres said.
"In the name of humanity, do not allow to start in Europe what could be the worst war since the beginning of the century."
The US ambassador to the United Nations, Linda Thomas-Greenfield, warned that an all-out Russian invasion could displace five million people, triggering a new European refugee crisis.
Living in fear
Western nations said ahead of Thursday's operation Russia had amassed 150,000 troops in combat formations on Ukraine's borders with Russia, Belarus and Russian-occupied Crimea and on warships in the Black Sea.
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.
Shelling had intensified in recent days between Ukrainian forces and Russia-backed separatists - a Ukrainian soldier was killed on Wednesday, the sixth in four days - and civilians living near the front were fearful.
Dmitry Maksimenko, a 27-year-old coal miner from government-held Krasnogorivka, told AFP that he was shocked when his wife came to tell him that Putin had recognised the two Russian-backed separatist enclaves.
"She said: 'Have you heard the news?'. How could I have known? There's no electricity, never mind internet. I don't know what is going to happen next, but to be honest, I'm afraid," he said.
Russia has long demanded that Ukraine be forbidden from ever joining the NATO alliance and that US troops pull out from Eastern Europe.
Speaking to journalists, Putin on Tuesday 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.
Washington Wednesday announced sanctions on the Nord Stream 2 gas pipeline, which Germany had earlier effectively suspended by halting certification.