Church bells rang and mourners laid flowers at Chernobyl's memorial square as the clock turned 1.23 am -- the moment the plant's reactor number four exploded and changed the fate of a generation living across the former Soviet Union.
"There was crying and screaming," local pensioner Maria Urupa told AFP as she recalled the terror that struck locals as they watched poisonous clouds of radiation waft in from the plant.
At least 30 people were killed on site and several thousand more are feared to have died from radiation in what Ukrainian President Petro Poroshenko said "appears to have been the world's largest man-made catastrophe".
More than 200 tonnes of uranium remain inside the crippled reactor that spattered radiation across three quarters of Europe after a botched safety test.
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Lingering fears of new leaks occurring should the ageing structure covering the toxins crack have prompted a global push to fund the construction of a giant new arch that should keep the site safe for generations.
The UN watchdog the International Atomic Energy Agency used the anniversary to warn against "complacency" in nuclear safety.
But Poroshenko said his country "will not be able to stop using atomic power" as it tries to wean itself off its energy dependence on its former master Russia.
Poroshenko also warned that Russia's alleged support for a pro-Moscow insurgency in the east was threatening a repeat of the Chernobyl catastrophe -- because Ukraine has its largest nuclear plant on the very edge of the war zone.
"Do not forget that warfare was being waged only a few kilometres from the Zaporizhia nuclear power plant," Poroshenko said after laying flowers at the dark grey stone plaques honouring those who died trying to contain the 1986 inferno at Chernobyl.