Ukraine has begun 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.
A memorial service was held in the town of Slavutych, built to re-house workers who lived near the nuclear plant.
Ukraine's President Petro Poroshenko is due to attend a ceremony near the site and a church service will be held in Kiev for the families of victims.
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 border are 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. It is thought that about 30 people died in the initial meltdown and rescue operation, and a UN report published in 2005 estimated that up to 4,000 people could eventually be killed by related illnesses.
But Greenpeace has said the UN figure is a underestimate.