Black-clad Orthodox priests sang solemn hymns, Ukrainians lit thin wax candles and a bell tolled 25 times for the number of years that have passed since the Chernobyl disaster as the world began marking the anniversary Tuesday of the worst nuclear accident in history.

A boy looks at a portrait of his grandmother as he visits a memorial dedicated to firefighters and workers who died after the Chernobyl nuclear disaster during a night service near the Chernobyl plant in the city of Slavutych April 25, 2011. Belarus, Ukraine and Russia will mark the 25th anniversary of the Chernobyl disaster, the world's worst civil nuclear accident.[Photo/Agencies]

Russian Orthodox Patriarch Kirill led the service near a monument to firefighters and clean up workers who died soon after the accident from acute radiation poisoning.

"The world had not known a catastrophe in peaceful times that could be compared to what happened in Chernobyl," said Kirill, who was accompanied by Ukraine's Prime Minister Mykola Azarov and other officials.

"It's hard to say how this catastrophe would have ended if it hadn't been for the people, including those whose names we have just remembered in prayer," he said in an emotional tribute to the workers sent to the Chernobyl plant immediately after one of its reactors exploded.

Tuesday's service began shortly after midnight, the same time as the blast on April 26, 1986, that spewed a cloud of radioactive fallout over much of Europe and forced hundreds of thousands from their homes in the most heavily hit areas in Ukraine, Belarus and western Russia.

The disaster did not become public knowledge for several days, because officials released no information until 72 hours after the accident.

The explosion released about 400 times more radiation than the US atomic bomb dropped over Hiroshima. Hundreds of thousands were sickened and once-pristine forests and farmland still remain contaminated. The UN's World Health Organization said at a conference in the Ukraine capital Kiev last week that among the 600,000 people most heavily exposed to the radiation, 4,000 more cancer deaths than average are expected to be eventually found. 

Several hundred Ukrainians, mostly widows of plant workers, came to Tuesday's nighttime service to pay their respects to their loved ones and colleagues. Teary-eyed, they lit candles, stood in silence and crossed themselves to the sound of Orthodox chants.

"Our lives turned around 360 degrees," said Larisa Demchenko, 64. She and her husband both worked at the plant, and he died nine years ago from cancer linked to Chernobyl radiation.