A fire broke out at the Zaporizhzhia nuclear plant, the largest in Europe after it was attacked by Russia.
Authorities say the facility is now safe and radiation levels are normal.
Ukrainian President Volodymyr Zelenskyy accused Russia of resorting to “nuclear terror” and wanting to repeat the 1986 Chernobyl disaster.
“If there is an explosion, it is the end of everything. The end of Europe,” he said.
A video feed from the nuclear plant showed blasts lighting up the night sky and sending up plumes of smoke.
Workers at the plant said the fire broke out at a training building outside the plant’s perimeter, and that only one of the plant’s six reactors were operational.
The UN’s nuclear watchdog, the International Atomic Energy Agency (IAEA), said the fire had not affected the plant’s “essential” equipment and there was no increase in radiation levels.
But the IAEA said it was in “full 24/7 response mode” due to the “serious situation” at the power station.
Ukrainian emergency services said initially they were blocked from getting to the scene of the blaze, prompting President Biden to publicly call on Russia to allow firefighters into the site.
But UK Prime Minister Boris Johnson said the “reckless” attack could “directly threaten the safety of all of Europe”.
US President Joe Biden urged Moscow to stop its military activities around the site, while Canada’s Prime Minister Justin Trudeau said the “horrific attacks” from Russia “must cease immediately”.
The plant, located around 550km south-east of the capital Kyiv, generates almost a quarter of all electricity in Ukraine.
Russian forces have already seized control of the Chernobyl nuclear plant, the site of the worst nuclear disaster in history.
More than one million people have fled Ukraine since the invasion began last week.
Despite punishing international sanctions, Russia’s President Vladimir Putin said his offensive was going “strictly according to schedule, according to plan”.
He has put his nuclear forces on high alert because of “aggressive statements” by the West.
The Europe’s largest nuclear plant in Zaporizhzhia was attacking the whole night by Russian forces. It’s serious! We should not let the environmental catastrophe happened! Actions should be taken! Please, #StopPutinNOW #SupportUkraine
Video source: @ZelenskyyUa pic.twitter.com/tGM4DeemDk
— Daniel Tonkopiy🇺🇦 (@DTonkopiy) March 4, 2022