Ukraine marked the 20th anniversary of the Chernobyl disaster Wednesday with pealing bells, candlelit ceremonies and parliamentary debates about how much the country still does not know - and how much it needs to do to bring relief to the millions affected by the world's worst nuclear accident. President Viktor Yushchenko, arriving by helicopter at the shuttered Chernobyl nuclear power plant, laid two red carnations beneath a monument to the victims of the April 26, 1986, explosion as dozens of the emergency workers who risked their lives stood nearby. "As I look into your eyes, I remember all those heroes who died 20 years ago for our lives, for our future," Yushchenko said. "Your feats will be remembered forever." The explosion of Chernobyl's Reactor No. 4 spewed clouds of radiation over large swathes of the Soviet Union and Europe. It forced the Soviet government to evacuate more than 300,000 people, leaving behind dozens of empty villages to rot and decay. The accident cast a radioactive shadow over the health of millions of people, spooked the world and, many believe, contributed to the Soviet Union's collapse half a decade later. About 5 million people live in areas covered by the radioactive fallout in Ukraine, Belarus and Russia. "Let God not allow this to be repeated. Let God not make our grandsons relive this," Valentyna Mashina, 55, said at a monument to victims in the town of Chernobyl 18 kilometers (11 miles) from the plant, where 4,000 people still live - but for no more than two weeks at a time - to work in the most highly contaminated zone. After 20 years, with international experts saying radiation levels are decreasing a hundredfold in areas around the blast, the United Nations has turned its attention to returning life and hope to the region _ and removing the malaise and sense of doom that it warned has left millions thinking of themselves as victims instead of survivors. "Chernobyl must not be a mourning place, it must become a place of hope," Yushchenko said. Dozens gathered in the town of Chernobyl for reunions with old friends, and parliament opened a special session dedicated to the accident. Many lawmakers demanded more help for the millions affected and more information to answer the still lingering questions over the accident's aftermath. Death toll numbers remain hotly disputed. One plant worker was killed instantly in the fire and explosion and his body has never been recovered. Twenty-nine rescuers, firefighters and plant workers died later from radiation poisoning and burns they suffered trying to contain the explosion and keep it from spreading to the plant's three other reactors. One more died from an apparent heart attack. Mykola Malyshev, 66, was working in the control room of Chernobyl's Reactor No. 1 at the time of the explosion. He said the lights flickered and the room shook. The workers were ordered to the destroyed reactor, but when they got there, their co-workers ordered them to flee and save themselves. "They told us, 'We are already dead. Go away,"' Malyshev recalled at the Kiev ceremony. Thousands have been diagnosed with thyroid cancer, and the U.N. health agency said about 9,300 people were likely to die of cancers caused by radiation. Dr. Mike Repacholi, the World Health Organization's radiation and environmental health chief, said at the end of a three-day conference in Kiev that there were some areas that needed further study, such as the effect of radioactive iodine on adults, as well as "the possibility of leukemia occurring at a later time and solid cancers that could appear some 20 to 25 years after the accident." Solid cancers typically refer to non-blood cancers. Some groups, including Greenpeace, have warned that death tolls could be 10 times higher than U.N. predictions, accusing it of whitewashing the impact of the accident as a bid to restore trust in the safety of atomic power. In Moscow's Red Square, 13 Greenpeace activists wearing T-shirts that collectively said "No more Chernobyls" chained themselves to a fence surrounding St. Basil's Cathedral. Police cut their chains and detained them as well as journalists covering the unsanctioned protest. At the Mitinskoye cemetery on Moscow's outskirts, where the initial victims are buried in lead-encased coffins, officials and priests joined dozens of relatives who wiped away tears at a solemn ceremony. Early Wednesday, in Kiev and Slavutych, a town built to house displaced Chernobyl workers, hundreds carrying red carnations and flickering candles filed by memorials, as bells tolled and sirens sounded to mark the exact time of the explosion - 1:23 a.m. The last working reactors at Chernobyl were closed in 2000.