Russian President Vladimir Putin demands Kyiv lay down arms and warns foreign powers not to interfere
Russian forces have launched a major military assault on Ukraine, with reports of explosions near major cities across the country.
In a pre-dawn TV statement Russian President Vladimir Putin said Russia did not plan to occupy Ukraine, but said Moscow's response would be "instant" if anyone tried to stop this.
Shortly afterwards, attacks were reported on Ukrainian military targets.
Ukraine said that "Putin has launched a full-scale invasion of Ukraine".
In a video statement on Thursday morning, Ukrainian President Volodymyr Zelensky said martial law was now being imposed across all of Ukraine.
"No panic. We're strong. We're ready for anything. We'll defeat everyone, because we are Ukraine," the president said.
He said Russia carried out strikes on Ukraine's military infrastructure and border guard units. Military headquarters, airfields and military warehouses in Kyiv, Dnipro and Kharkiv, officials said.
Reports are also coming of explosions in several Ukrainian cities, and gunfire near main Boryspil airport in the capital Kyiv. Warning sirens were heard blaring in the city with a population of about three million.
In a last-ditch attempt to avert war shortly before Mr Putin's announcement, Mr Zelensky had warned that Russia could start "a major war in Europe" and urged Russian citizens to oppose it.
Mr Zelensky said Russia had almost 200,000 troops and thousands of combat vehicles on Ukraine's borders.
Ukraine's Western allies had also repeatedly warned Russia was poised to invade.
Responding to Russia's assault, US President Joe Biden said Washington and its allies would respond in a united and decisive way to "an unprovoked and unjustified attack by Russian military forces" on Ukraine.
"President Putin has chosen a premeditated war that will bring a catastrophic loss of life and human suffering," Mr Biden said. "The world will hold Russia accountable."
UK Prime Minister Boris Johnson said he was "appalled by the horrific events in Ukraine" and that President Putin "has chosen a path of bloodshed and destruction by launching this unprovoked attack". He added that he had spoken to Ukraine's president to discuss how to respond and promises decisive action by the UK and allies.
Russia launched its military operation just days after recognising the self-proclaimed people's republics of Donetsk and Luhansk in eastern Ukraine.
The breakaway regions - who control large swathes of the Donbas region - later asked Moscow for military support.
In his announcement early on Thursday, Mr Putin said the military operation's objective was to defend those people who had been subjected for eight years to "genocide by the Kyiv regime".
It was an apparent reference to mass street protests in Ukraine that ousted Pro-Russian President Viktor Yanukovych in 2014.
Mr Putin also said that Moscow would seek "demilitarisation and denazification" of Ukraine.
Kyiv and its Western allies have repeatedly rejected as absurd Mr Putin's claims that Ukraine was being run by neo-Nazis, instead pointing that - unlike an authoritarian Russia - Ukraine was now a nation with growing democratic institutions.
The first explosions were heard here in Kyiv shortly after five in the morning. There were four or five, muffled and distant.
Others followed soon afterwards, the most recent sounded closer but still not in the city centre.
One report suggested the airport might be under attack. A government official dealing with the foreign press said airfields and military headquarters had been attacked.
Other reports have been coming in from across Ukraine, including towns in the east, like Kramatorsk, close to areas controlled by pro-Russian separatists.
Coming shortly after Vladimir Putin's latest speech, this appears to mark the start of Russia's long-awaited attack.
Fears of a Russian attack have been rising for months.
Mr Putin has repeatedly accused the US and its allies of ignoring Russia's demands to prevent Ukraine from joining Nato military alliance and offer Moscow security guarantees.
On Wednesday, Ukraine's state of emergency was overwhelmingly approved by lawmakers, and is now in force nationwide.
It introduces personal document checks, blocks military reservists from leaving the country, bans mass gatherings and places restrictions on radio communication systems. Kyiv's mayor said checkpoints would be set up in roads into the city, and access to government buildings would be restricted.
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