CHERNOBYL/VIENNA: Ukrainians held candlelit vigils on Tuesday to mark 30 years since the world’s worst nuclear accident at Chernobyl spewed radiation across Europe and left several thousand people dead or dying.

Church bells rang and mourners laid flowers at Chernobyl’s memorial square as the clock turned 1:23am — 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 said 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”.

The exact number of dead remains a subject of intense debate because the Soviet authorities kept most of the information about the disaster hidden. More than 200 tonnes of uranium remain inside the crippled reactor that spattered radiation across three quarters of Europe after a botched safety test.

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 atomic watchdog used the 30th anniversary on Tuesday of the Chernobyl disaster, the world’s worst nuclear accident, to warn against “complacency” in atomic safety.

“Chernobyl led to a leap forward in global cooperation on nuclear safety,” International Atomic Energy Agency chief Yukiya Amano said in a statement. Further improvements were made after the Fukushima disaster in Japan in 2011, but the “key lesson” from both events “is that safety can never be taken for granted”. “Complacency must be avoided at all costs,” he said.

He said nuclear safety is primarily the responsibility of individual states, but that since accidents can transcend borders, effective international cooperation is “vital”.

After Chernobyl, Amano said, countries with nuclear power began sharing information and experience in a way they never had before. The IAEA’s mandate on nuclear safety was enhanced and safety standards were expanded.

Published in Dawn, April 27th, 2016