Russia has launched a “full-scale invasion of Ukraine”, the Ukrainian foreign minister has said.
Russian President Vladimir Putin told his citizens he had begun a “specialised military operation”.
Where is military action taking place?
Shortly after Mr Putin made his announcement on state television, just before 3am GMT, large explosions were heard in Kyiv, Kharkiv, Odessa and other cities across Ukraine.
In the capital Kyiv, gunfire could be heard near the airport, the Russian Interfax news agency said.
Ukrainian media reported that military command centres there had been struck by missiles.
Sky News’s Stuart Ramsay, who is in Kyiv, said he was woken up by loud explosions.
“There are major attacks happening,” he said. Air raid sirens were heard going off in the city.
In Kharkiv, in the northeast of the country, command centres were also hit by missiles, Ukrainian media reported.
Explosions could be heard, a witness told the Reuters news agency, and the windows in his apartment block had been shaking.
Many people are trying to leave.
Ukraine said Russia was shelling across country all the way to the western Lviv region, 70 km from the border with Poland.
Russia is also moving military equipment into Ukraine from annexed Crimea, Ukrainian authorities said.
And in the southern port cities of Odessa and Mariupol, Russian troops have reportedly landed.
A witness heard three loud blasts in Mariupol, Reuters said.
According to Interfax, Russian-backed separatists said they had launched an offensive on the Ukrainian-controlled town of Shchastia in the east.
Explosions are also said to have rocked the breakaway eastern Ukrainian city of Donetsk.