"The Chernobyl catastrophe does not have a uniquely Ukrainian character but touches many peoples," President Viktor Yushchenko of Ukraine, where the defunct power plant is located, said this week.

At a ceremony for survivors in Moscow, Russian President Vladimir Putin honoured the "liquidators" who cleaned up after the tragedy, saying: "Your selflessness and ability to face an extreme situation saved a large number of human lives."

Commemoration ceremonies were due to begin in Ukraine late Tuesday near the reactor at the power plant that was ripped apart by two explosions at 1:23 am local time on April 26, 1986, spewing a huge radioactive cloud into the air.

Candlelight vigils were to take place early Wednesday in the town of Slavutich, home to most of the 3,700 workers who still service the plant, and in Kiev, where many of the "liquidators" live today.

In Belarus, where much of the radioactive cloud settled after the accident, opposition groups were expected to hold what has become a traditional protest Wednesday against government efforts to repopulate the affected areas of the country.

Critics say that Belarus's authoritarian leadership is ignoring serious health risks in trying to return the contaminated land back to general use.

Belarus lies less than 40 kilometers (25 miles) to the north of the station and nearly a quarter of its land was contaminated as a result of the Chernobyl accident.

But the radioactivity also drifted over a large swathe of Europe and its effects were felt from Scandinavia to Greece.

The Soviet leadership kept silent immediately after the disaster -- which occurred less than 100 kilometers from a major city, Kiev, and near the Dniepr River that provides much of Ukraine's drinking water -- and the world did not learn of the accident until days later.

Some five million people are believed to have been affected by the disaster in Belarus, Russia and Ukraine, all of which still have regions where the levels of dangerous cesium-137 and strontium-90 radioisotopes are much higher than those normally accepted.

Nearly 800,000 hectares (3,100 square miles) of prime agricultural land and 700,000 hectares (2,700 square miles) of forest remain derelict in the three countries.

"The legacy of Chernobyl is very much with us today, even after two decades," Kalman Mizsei, regional director for the UN Development Program, said this week.

The death toll from the tragedy is hotly debated, two decades on.

The United Nations, backed by the governments of Belarus, Russia and Ukraine, said in a report last September that some 4,000 people could be expected to die overall as a direct consequence of the accident -- a much lower figure than previously thought.

The UN's World Health Organization (WHO) later upped its estimate to 9,000 deaths.

But the Greenpeace environmental group has dismissed the findings as a "whitewash" and in a recent report of its own said that the death toll could reach nearly 100,000. Other anti-nuclear groups have come up with higher estimates.

Chernobyl has gobbled up astronomic amounts of international funds.

Ukraine estimates it will spend up to 170 billion dollars (137 billion euros) by 2015; Belarus says its losses over the past two decades have come to 235 billion dollars; the UN says the accident will end up costing hundreds of billions of dollars overall.

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 the construction of a 20,000-ton steel case to take its place.

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