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 and its Western allies earlier said Russia was poised to invade.
Reports are now coming of explosions in several Ukrainian cities, and gunfire near Kyiv’s main Boryspil airport.
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.
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.
Fears of a Russian attack have been rising for months.
Ukraine’s state of emergency was overwhelmingly approved by lawmakers on Wednesday, 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.