The April 26, 1986, catastrophe at the nuclear power plant in then-Soviet Ukraine sent a radioactive cloud over much of Europe and contaminated large areas of Ukraine, Russia and Belarus.
"It is impossible to reconcile ourselves to this. We are a sick nation and we must do everything to become a normal nation," opposition politician Pavel Severinets said at an evening commemoration rally in the Belarusian capital of Minsk.
Earlier Thursday, declaring the world should "never forget Chernobyl," Secretary-General Kofi Annan said in a message: "Together we must extend a helping hand to our fellow human beings and show that we are not indifferent to their plight."
The appeal has been made annually for the past five years, with U.N. officials saying most monies are earmarked to help shut down the infamous power plant in the Ukraine and construct a confinement structure, called a sarcophagus, to contain the remains of the reactor core.
"While hundreds of millions of dollars have been pledged to the construction of a new and safer sarcophagus, comparatively little has has been done by the international community to provide direct assistance to the population affected," said Kenzo Oshima, the U.N. undersecretary-general for humanitarian affairs.
The mother of a firefighter
who died in the clean-up
wept on the 15th anniversary
of the Chernobyl fire (AP)SIZE>
In all, 7 million people in Belarus, Russia and Ukraine are estimated to suffer physical or psychological effects of radiation related to the Chernobyl catastrophe.
Hundreds of people attended an overnight memorial service at a small Kiev chapel that was built to commemorate the disaster. They held burning candles as priests read out prayers in memory of the dead.
A bell rang shortly after 1 a.m., exactly the same time as the reactor exploded. Some in the crowd broke into tears.
One woman described how the building in which she worked at Chernobyl grew dark and shook. From a window, she saw "a glow, like haze in the summer" over the reactor.
A similar service was held in Slavytuch, a town of Chernobyl workers close to the plant. President Leonid Kuchma was due to visit the plant later in the day.
In a statement marking the anniversary, Kuchma urged the world not to forget Chernobyl and to help Ukraine deal with its consequences.
"Chernobyl is a common tragedy, common pain of our planet, and its echo must not fall silent in our hearts," Kuchma said.
Boris Chekalin, the head of the radiation service at Russia's Kursk atomic power plant, took part in the Chernobyl cleanup. He told Russian state television about the first days of the operation.
"When I arrived at Chernobyl, I saw a large black fire with clouds, an impression that will stay with me my whole life," he said.
Chekalin said he never takes off his hat, even on overcast days, because he has to avoid the most minor sun rays to prevent irritating burns on his face and arms a constant reminder of his radiation exposure during three days at Chernobyl.
Following the 1986 explosion, other reactors at the plant continued operating until it was halted for good in December under intense international pressure.
At the plant itself, workers remain busy. They monitor the now-idle reactors and are building a heating plant and facilities for nuclear waste disposal and reprocessing.
They are also involved in a $758 million, internationally funded project to make the leaky concrete and steel sarcophagus over the ruined reactor environmentally safe.
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