Twenty years after an explosion ripped the roof off Chernobyl's Reactor 4, the Ukrainian Government today pledged to begin rebuilding the lives of those still living in the shadow of the world's worst nuclear disaster.

As crowds returned to the abandoned city whose name has become synonymous with suffering, ministers promised to address the malaise that many claim has destroyed more lives in the region than radiation.

Meanwhile, protesters in Minsk, Moscow, Madrid, Istanbul and around the world used the anniversary as an opportunity to remind the world of the dangers of nuclear power, now making a comeback as a cheaper and cleaner alternative to oil and gas.

At the Vatican Pope Benedict XVI led his weekly public audience in St Peter's Square with prayers for the victims and a call for responsibility in the use of nuclear energy. He urged world leaders to ensure "with a concerted effort, that all energy is put to the service of peace, with respect for the needs of man and nature."

At 1.23am local time today, exactly 20 years after Reactor 4 exploded belching a radioactive cloud over Eastern Europe, bells tolled and sirens sounded in cities across Ukraine. In Kiev and Slavutych, a town built to house the displaced citizens of Chernobyl, hundreds of people carrying red carnations and flickering candles filed past memorials to the dead.

At the focus of today's commemorations in Chernobyl, a city now virtually deserted, the mood among the crowd was sombre.

President Viktor Yushchenko flew in a helicopter over the remains of the reactor before laying two carnations at a memorial to the liquidators - the young men who died from the blasts of radiation as they smothered the smouldering plant with concrete.

He told the crowd: "I look into your faces and I remember Ukrainian heroes who 20 years ago stopped the threat to our future. This heroic feat will be with us forever but Chernobyl must not be a mourning place, it must become a place of hope."

He continued: "The trance we were left in by Chernobyl is over. We are a strong and brave people and we are looking to the future."

Hundreds laid flowers at a memorial wall engraved with the names of those heroes.

"I knew all of these people," a tearful Mykola Ryabushkin told the AFP news agency, pointing to the portraits hanging on the monument. "I look at them and I want to ask them for forgiveness," he said. "Maybe we're all to blame for letting this accident happen."

Mykola Malyshev, 66, was working in the control room of Reactor No 1 at the time of the explosion. He remembered how his workmates were ordered to join the emergency effort within the destroyed reactor. On arrival, he said, their co-workers ordered them to flee. "They told us 'we are already dead. Go away, flee, save yourselves'," he recalled.

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