Commemorations are under way in Ukraine to mark the 30th anniversary of the nuclear disaster at Chernobyl.
Sirens were sounded at the same moment as the first explosion at the reactor, in the early hours of 26 April 1986.
The meltdown at the Soviet plant was the worst nuclear disaster in history.
An uncontrolled reaction blew the roof off, spewing out a cloud of radioactive material which drifted into other parts of the USSR, including Russia and Belarus, and northern Europe.
Relatives of those who died attended candle-lit vigils at several churches, including at Slavutych, a town built to re-house workers who lived near the nuclear plant. A series of events are being held throughout the day.
Ukrainian President Petro Poroshenko laid a wreath and observed a minute’s silence in the Ukrainian capital Kiev before heading north for a ceremony at the plant itself, not far from the Belarussian border.
Speaking in Chernobyl, he said the nuclear disaster had been Ukraine’s biggest challenge between the Nazi occupation in World War Two and the recent conflict in eastern Ukraine, which he described as “Russian aggression”.
“At a time when we still need immense resources to tackle the consequences of the Chernobyl disaster, when we need funding for social support to fire-fighters and victims, we have to spend almost one-fifth of our budget expenses on defence and security,” he said.
Vasyl Markin, who had been working in Chernobyl at the time of the disaster, attended the midnight vigil in Slavutych.
“This tragedy will stay with us till the end of our lives,” he said. “I won’t be able to forget it anyway.”
Some former residents returned to the area, now derelict and overgrown, ahead of the anniversary.
Zoya Perevozchenko, 66, had lived in Pripyat, the town inhabited by Chernobyl workers which was abandoned in the wake of the accident.
She told Reuters news agency: “I barely found my apartment, I mean it’s a forest now – trees growing through the pavement, on the roofs. All the rooms are empty, the glass is gone from the windows and everything’s destroyed.”
Levels of radioactivity remain high in the surrounding area. A charity, Bridges to Belarus, is warning that a number of babies in a region close to Ukraine’s border are still being born with serious deformities, while an unusually high rate of people have rare forms of cancer.