KIEV, Ukraine (AP) — With flowers, candles and tears, Ukraine on Tuesday marked the 30th anniversary of the explosion at the Chernobyl nuclear plant, the world's worst nuclear disaster. Some survivors said the chaos of that time is etched in their minds forever.

Ukrainian President Petro Poroshenko led a ceremony in Chernobyl, where work is underway to complete a 2 billion euro ($2.25 billion) long-term shelter over the building containing Chernobyl's exploded reactor. Once the structure is in place, work will begin to remove the reactor and its lava-like radioactive waste.

"We honor those who lost their health and require a special attention from the government and society," Poroshenko said. "It's with an everlasting pain in our hearts that we remember those who lost their lives to fight nuclear death."

About 600,000 people, often referred to as Chernobyl's "liquidators," were sent in to fight the fire at the nuclear plant and clean up the worst of its contamination. The initial explosion on April 26, 1986, at the power plant killed at least 30 people, exposed millions to dangerous levels of radiation and forced a wide-scale, permanent evacuation of hundreds of towns and villages.

But since the Ukrainian government has scaled back benefits for Chernobyl survivors, many of them feel betrayed by their own country.

"I went in there when everyone was fleeing, we were going right into the heat," said Mykola Bludchiy, who arrived in the Chernobyl exclusion zone on May 5, just days after the explosion. "And today everything is forgotten. It's a disgrace."

He spoke Tuesday after a ceremony in Kiev, where top officials were laying wreaths to a Chernobyl memorial.

At midnight on Monday, a vigil was held in the Ukrainian town of Slavutych, where many former Chernobyl workers were relocated.

Thirty years later, many could not hold back the tears as flowers and candles were brought to a memorial to the workers killed in the explosion. Some of the former liquidators dressed in white robes and caps for the memorial, just like those they were wearing in the aftermath of the disaster.

Andriy Veprev, who had worked at the Chernobyl nuclear plant for 14 years before the explosion and helped to clean up the contamination, said memories of the mayhem in 1986 were still vivid.

"I'm proud of those guys who were with me and who are not with us now," he said.

The final death toll from Chernobyl is subject to speculation, due to the long-term effects of radiation, but ranges from an estimate of 9,000 by the World Health Organization to a possible 90,000 by the environmental group Greenpeace.

In Russia, President Vladimir Putin said in a message to the liquidators released by the Kremlin press office that the Chernobyl disaster was "a grave lesson for all of mankind."

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Nataliya Vasilyeva contributed to this report from Moscow.