On 26 April 1986, the No. 4 reactor at the Chernobyl plant, then in the Soviet Union, exploded and caught fire after a safety test went badly wrong.
The blast sent radiated clouds billowing across Europe.
A total of 31 people died immediately but many more died of radiation-related sicknesses such as cancer, many of them in what is today's Belarus.
Tens of thousands were evacuated, never to return, from Prypyat, the town closest to the site which then had a population of 50,000.
Last week the world community, spurred by the nuclear crisis at Fukushima, pledged €550 million to help build a new containment shell over the stricken reactor in Chernobyl to replace a makeshift one that has again begun to leak radiation.
"Chernobyl was a challenge of planetary dimensions. The answer to this challenge can be provided only by the world community," Ukrainian President Viktor Yanukovich said.
Yanukovich was expected to visit Chernobyl with Russian President Dmitry Medvedev and Russian Orthodox Patriarch Kirill today (26 April), to commemorate the tragedy.
Although Chernobyl town itself was relatively untouched by the accident, Prypyat is now a ghost town at the centre of a largely uninhabited exclusion zone with a radius of 30 km (19 miles).
On 12 April, Japan raised the severity rating at its stricken Fukishima plant to seven, the same level as that of Chernobyl.
The nuclear crisis has put increasing pressure on the Japanese prime minister Naoto Kan to step down, and spurred worldwide protests.
On Monday, thousands of people in Austria, France and Germany staged marches calling for an end to nuclear power.
At the Pont de l'Europe bridge, linking France and Germany over the Rhine between Strasbourg and Kehl, demonstrators waved banners and chanted: "Chernobyl, Fukushima, never again!"
Their key demand was for the closure of France's oldest nuclear power station at Fessenheim.
As a siren wailed, they threw flowers on to the Rhine and lay down on the bridge in a symbolic "die-in".
In Ukraine too, the atmosphere was mournful.
"This is a day of mourning for us. We are in mourning for the people who 25 years ago fought to protect us," said Gennady Pikul, 50, referring to firefighters and other 'liquidators' who risked their lives to control the blazing reactor.
"We will do everything we can so that this is never repeated," he said.
In 1986, Soviet officials withheld reporting of the accident for two days, provoking accusations of a cover up.
"I think that our state must learn the lessons from what happened," Medvedev said, "from the now-distant Chernobyl incident in 1986 and the recent tragedies in Japan. Perhaps the most important lesson is the need to tell people the truth."
Because of the world's fragility and inter-connectedness, downplaying such crises resulted in a loss of human lives, he added.
(EurActiv with Reuters.)