Ukraine is marking the 25th anniversary of the world's worst nuclear accident at the Chernobyl power plant.
An explosion at one of the plant's reactors sent a plume of radiation across Europe in 1986, harming or killing possibly thousands of people.
According to the BBC, Ukrainian President Viktor Yanukovych and his Russian counterpart, Dmitry Medvedev, are visiting the site for a memorial ceremony. Early on Tuesday, Yanukovych attended a candle-lighting service led by Russian Orthodox Patriarch Kirill in the Ukrainian capital, Kiev.
"The world had not known a catastrophe in peaceful times that could compare to what happened in Chernobyl," Patriarch Kirill said. A bell sounded at 0123 (2223 GMT Monday), the time of the blast, and tolled 25 times.
It was on 26 April 1986 that Number Four reactor at Chernobyl, which was then in the Soviet Union, exploded. The accident forced the evacuation of hundreds of thousands of people from their homes in Ukraine, western Russia and Belarus.
Soviet officials held off reporting the accident for several days. There is still a 30 km (19 mile) exclusion zone around the plant.
Soviet engineers encased the damaged reactor in a temporary concrete casing to limit the radiation but a new shield is needed. A donors conference in Kiev, Ukraine, last week raised EUR550m ($798m) of the EUR740m needed to build a new shelter and a storage facility for spent fuel.
The Chernobyl anniversary comes less than two months after the Fukushima nuclear plant in Japan was badly damaged by an earthquake and tsunami, renewing concerns about the safety of nuclear power generation. The operator of the Fukushima plant, Tokyo Electric Power, has also come under fire for not quickly disclosing information on radiation leaks from the plant.
Medvedev said there must be greater transparency in nuclear emergencies. "I think that our modern states must see the main lesson of what happened at Chernobyl and the most recent Japanese tragedy as the necessity to tell people the truth," he told survivors of the clean-up effort at a meeting in the Kremlin.
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