Ukraine is holding commemorations to mark the 30th anniversary of the nuclear disaster in 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 plant remains the worst nuclear disaster in history.
An uncontrolled reaction blew the roof off, spewing out a cloud of radioactive material which drifted across Ukraine's borders, into Russia, Belarus and across a swathe of northern Europe.
The relatives of those who died attended candle-lit vigils at several churches, including in the capital Kiev and in Slavutych, a town built to re-house workers who lived near the nuclear plant.
"This tragedy will stay with us till the end of our lives. I won't be able to forget it anyway," said Vasyl Markin, who had been working in Chernobyl at the time of the disaster, and attended the service in Slavutych.
Some former residents returned to the area, now derelict and overgrown, ahead of the anniversary.
Zoya Perevozchenko, 66, 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 borderare still being born with serious deformities, while an unusually high rate of people have rare forms of cancer.
Donors around the world pledged €87.5m (£68m; $99m) on Monday towards a new underground nuclear waste facility in the region. Ukraine will need to commit a further €10m in order to complete the new storage site.
Work began in 2010 on a 25,000-tonne, €2.1bn sarcophagus to seal the uranium left in the damaged reactor, thought to be about 200 tonnes.
Experts fear that if parts of the aging reactor collapse, further radioactive material could be spewed into the atmosphere.
The number of people killed by the disaster remains disputed. A report in 2005 by the UN-backed Chernobyl Forum concluded that fewer than 50 people died as a result of exposure to radiation, most of them workers killed immediately after the disaster, but some survived until as late as 2004.
The forum estimated up to 9,000 people could eventually die from radiation exposure, although Greenpeace claims the figure could be as high as 93,000.