A fire broke out at the Zaporizhzhia nuclear plant, the largest in Europe, and Ukraine said it was shelled by Russian troops.
Authorities say the facility is now safe and radiation levels are normal.World leaders have accused Russia of endangering the safety of an entire continent, and Ukraine's president accused Russia of "nuclear terror".
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".
UK Prime Minister Boris Johnson
said the "reckless" attack could "directly threaten the safety of all of Europe". All three leaders spoke to Ukraine's President Volodymyr Zelensky by phone.
Zelensky, meanwhile, said Russia wanted a repeat of Chernobyl, the site of the world's worst nuclear disaster in 1986.
"If there is an explosion, it is the end of everything. The end of Europe," he said.
Russia's defence ministry blamed the attack on Ukrainian saboteurs, calling it a "monstrous provocation" without providing evidence.
A video feed from the nuclear plant showed blasts lighting up the night sky and sending up plumes of smoke.
Buildings around one of the plant's six power units have been damaged without affecting its safety, according to Ukraine's nuclear inspectorate.
Workers at the plant said the fire - which has since been extinguished - broke out at a training building outside the plant's perimeter, and that only one of the plant's six reactors was operational.
One resident who lives nearby said he saw the Russian military attacking the site. "It is just terrorism... It is worrying not just for our region, but for Ukraine and for the world," Kirill Dovzhik told the BBC.
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 they were initially blocked from getting to the scene of the blaze, prompting President Biden to publicly call on Russia to allow firefighters into the site.
said he would seek an emergency meeting of the UN Security Council on Friday over the attack.
Experts said attacking a nuclear plant was unprecedented, and the situation was still very dangerous.
Dr Graham Allison, a nuclear security expert at Harvard University, said the "worst case" would be if a fire at the plant caused a meltdown and prompted a release of radioactivity that contaminated the surrounding area for years.
But he also said it was more likely Russian forces were trying to "close down the supply of electricity to the surrounding area", rather than attack the plant.
The plant, located around 550km (342 miles) southeast of the capital Kyiv, generates almost a quarter of all electricity in Ukraine. It lies on the river Dnieper, northeast of the city of Kherson which is said to have been occupied by Russian forces.
The Russian military has also seized control of the Chernobyl nuclear plant, the site of the worst nuclear disaster in history.
News of the latest incident at Zaporizhzhia caused share prices in Asia to fall sharply.
In other developments:
In the port city of Mariupol in southern Ukraine, residents are without power and water supplies because of relentless Russian shelling
In the north, the cities of Chernihiv and Kharkiv have again come under fire, while there are reports that the city of Sumy has been surrounded by Russian troops
A second round of peace talks between Russia and Ukraine agreed to provide humanitarian corridors to evacuate civilians from conflict zones
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.
Western ministers will hold crisis talks in Brussels on Friday as they seek to show their support for Ukraine.
UK Foreign Secretary Liz Truss called it "one of the biggest days of diplomacy" with allies prepared to "tighten the vice around Putin's war machine" by targeting the Russian economy with more sanctions.