Russian President Vladimir Putin has launched a “special military operation” in Ukraine’s eastern Donbas region.
In a televised address, he urged Ukrainian soldiers who are facing off Russian-backed rebels to lay down weapons and return to their homes.
Mr Putin said Russia did not plan to occupy Ukraine, but warned that Moscow’s response would be “instant” if anyone tries to take on Russia.
Ukraine said that “Putin launched a full-scale invasion of Ukraine”.
In a tweet, Ukrainian Foreign Minister Dmytro Kuleba wrote: “Peaceful Ukrainian cities are under strikes. 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.”
Ukraine’s Western allies had earlier warned Russia was poised to invade.
Reports are now coming of explosions in several Ukrainian cities, and gunfire near main Boryspil airport in the capital Kyiv.
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.”
In a last-ditch attempt to avert war shortly before Mr Putin’s announcement, Ukrainian President Volodymyr Zelensky said 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.
Russia is launching 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, 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.
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.