President Viktor Yushchenko led hundreds of the plant's former and present employees in a minute's silence in front of reactor number four, ripped apart by two pre-dawn explosions on April 26, 1986.

"I look into your faces and I remember Ukrainian heroes who 20 years ago stopped the threat to our future," Yushchenko told the crowd, many clutching red carnations. "This heroic feat will be with us forever."

The blasts at the Soviet-era plant created a cloud of radioactive dust that drifted over a large swathe of Europe and still haunts millions of people in Ukraine and its neighbours.

Many claim the disaster played a key role in the fall of the Soviet Union, whose leadership concealed the extent of the accident from its own people and the world for several days. Evacuations from affected areas began only a day and a half after the disaster.

Away from Chernobyl, memorial services were held in the capitals of Belarus and Ukraine, the two nations most affected by the accident.

Pope Benedict XVI used the 20th anniversary to issue a plea for all energy to be used in the service of world peace, while environmental groups protested against the continued use of atomic energy.

In Belarus to the north, which bore the brunt of the radioactive fallout, 1,500 people attended a requiem mass for the clean-up workers deployed across the Soviet Union to deal with the disaster.

"We come each year on this day of sorrow," said Valentina Kaspirova, who attended the Minsk requiem holding a sign "Chernobyl: a tragedy for each and all."

The Chernobyl anniversary has become a traditional day of protests for the opposition in authoritarian Belarus, which planned to hold an evening rally in the capital.

The nation's leadership has sought to repopulate areas abandoned in the wake of the accident, but critics charge they are ignoring serious health risks in doing so.

Environmental groups staged small rallies in Kiev and in Moscow's Red Square protesting against construction of new atomic reactors, with Russian police briefly detaining a group of Greenpeace activists in Moscow.

For many of the 350,000 people eventually evacuated from contaminated zones around the plant, April 26 is a day for quiet remembrance.

"Today we're going home to Pripyat," said Liliya Vaganova, 50, referring to the city north of the defunct plant that once housed 45,000 people, but which today is a ghost town overrun by vegetation.

"It is a memory of better things. Pripyat was the best time of our lives," she said.

Some five million people are believed to have been affected overall by the disaster in Belarus, Russia and Ukraine, where millions of acres (hectares) of agricultural and forest land remain contaminated.

Two decades on, the disaster's death toll is hotly debated.

UN agencies estimate up to 9,000 people could be expected to die overall as a direct consequence of the accident, and that the disaster will end up costing hundreds of billions of dollars.

Environmental groups such as Greenpeace dismiss such figures as a whitewash and say up to 100,000 people could die.

The Chernobyl plant was eventually closed for good in December 2000 but will continue to be a concern for years to come.

The concrete sarcophagus that was hastily constructed over its destroyed reactor immediately following the accident is showing signs of wear, and more than 20 countries have chipped in nearly a billion dollars for construction of a 20,000-ton steel case to take its place.

Construction of the new containment unit is expected to begin later this year and Ukraine hopes to complete it by 2010.

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