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 April 26, 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 Union of Soviet Socialist Republics-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.
“We honour those who lost their health and require a special attention from the government and society. It’s with an everlasting pain in our hearts that we remember those who lost their lives to fight nuclear death,” President Poroshenko said in a speech in Chernobyl.
He also 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.
Russian President, Vladimir Putin, in a message to the 600,000 people who helped in the clear-up, known as liquidators, called the nuclear disaster “a grave lesson for all of mankind.”