Patriarch Kirill (left); Russian President Dmitry Medvedev; Vladimir, Metropolitan of Kyiv and all Ukraine; and Ukrainian President Viktor Yanukovych ring bells at the St. Ilyinski church in Chornobyl.
Ceremonies have been held across Ukraine to mark the 25th anniversary of the Chornobyl nuclear disaster.
Hundreds of mourners lit candles at an early morning memorial service conducted in Kyiv for the victims of the world's worst nuclear accident, and a bell in the capital tolled 25 times to mark the number of years that passed since the disaster.
Vigils were held in a number of Ukrainian cities including Slavutych, where many former Chornobyl employees now live.
Ukrainian President Viktor Yanukovych and Russian President Dmitry Medvedev laid red roses during a memorial ceremony outside the Chornobyl plant.
WATCH: After the Chornobyl disaster, some residents were relocated to a newly built town in eastern Ukraine. But some did not escape the radiation zone soon enough, and many have died prematurely:
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Far From Chornobyl, Evacuees Still Live With Its Impact
Yanukovych paid tribute to those who lost their lives cleaning up the contaminated wreckage in the wake of the explosion.
"We all bow our heads today in memory of those who were saving us and all mankind," he said. "We are grateful to them."
Medvedev said the need for government transparency was one of the lessons of the Chornobyl disaster.
"It is the duty of the state to tell people the truth. We have to admit that the state did not always act as it should have," he said, alluding to how Soviet authorities responded to the events at Chornobyl in 1986. "To avoid such tragedies, we should all be truthful and provide absolutely accurate information about what goes on -- whatever happens and wherever it happens."
Medvedev said he had sent world leaders proposals for a new global nuclear safety convention to ensure the disasters that struck Chornobyl and most recently Fukushima, in Japan, are not repeated.
But he said the two accidents did not mean the world should abandon nuclear power.
"No one has yet offered any other sources of energy that could replace nuclear energy. And that is probably not needed," Medvedev said. "Most importantly, we need to understand what kind of power mankind deals with so that our technological decisions match the challenges posed by nuclear technology."
On April 26, 1986, a safety test at the Chornobyl nuclear power plant went wrong and caused Reactor No. 4 to explode and catch fire.
Radioactive material spread over large swaths of what is today Ukraine, Belarus, and southern Russia, before being blown across Western Europe.
Ukrainian President Viktor Yanukovych at the Chornobyl power station this month
"As soon as we crossed the Prypyat bridge, we saw the power plant and we could clearly see the outside destruction," says Andriy Gusin, who worked the morning shift at the plant hours after the accident.
"When we started our shift we didn't yet know what kind of destruction it was, but it was clear it was important," he recalls.
Official figures put the death toll in the immediate aftermath at about 30 people. But independent health experts say many more have since died of radiation-related diseases such as cancer.
Soviet authorities took two days to report the accident, prompting accusations that the Soviet leadership had initially sought to cover up the disaster.
Hundreds of thousands of people were eventually evacuated, from the nearby town of Pripyat, which housed the plant's workers, and from other towns and villages within a radius of 30 kilometers.
The so-called exclusion zone today remains deserted, save for a handful of people who have chosen to return to their former homes despite the lingering threat of radiation.
Stricken Reactor Leaking Radiation
The power plant itself continued producing electricity until 2000, when Ukraine's then-President Leonid Kuchma bowed to international pressure and ordered the last operating reactor to be shut down.
Russian President Dmitry Medvedev (right) shakes hands with Ukrainian President Viktor Yanukovych at the memorial events in Chornobyl.
Speaking at the ceremony in Chornobyl, Medvedev said the disaster had shown the need for transparency in dealing with nuclear accidents.
"It is the duty of the state to tell people the truth. We have to admit that the state did not always act as it should have," he said. "And to avoid such tragedies, we should all be truthful and provide absolutely accurate information about what goes on -- whatever happens and wherever it happens."
The international community, spurred by the nuclear accident at Japan's Fukushima plant, last week pledged the equivalent of $780 million to help build a new encasement over Chornobyl's stricken reactor.
The shell of metal and concrete known as the "sarcophagus" has long exceeded its initial lifespan and now leaks radiation.
Yanukovych today renewed calls on the international community to donate the remaining $300 million needed to complete the project.
"Chornobyl was a challenge of planetary dimensions," he said in a statement on his website. "The answer to this challenge can be provided only by the world community."
written by Claire Bigg, with agency reports
The abandoned city of Pripyat near the Chornobyl nuclear power plant
