With flowers, candles, anger and tears, Ukraine on Tuesday marked the 30th anniversary of the explosion at the Chernobyl nuclear plant, the world's worst nuclear accident. 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 $2.25 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.

The disaster shone a spotlight on lax safety standards and government secrecy in the former Soviet Union. The explosion on April 26, 1986, was not reported by Soviet authorities for two days, and then only after winds had carried the fallout across Europe and Swedish experts had gone public with their concerns.

"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. Thirty workers died either from the explosion or from acute radiation sickness within several months. The accident exposed millions in the region to dangerous levels of radiation and forced a wide-scale, permanent evacuation of hundreds of towns and villages in Ukraine and Belarus.

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 one of a possible 90,000 by the environmental group Greenpeace.

The Ukrainian government, however, has since scaled back benefits for Chernobyl survivors, making many 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.

In neighboring Belarus, where over 470 towns and villages had to be permanently evacuated due to radiation from Chernobyl, opposition activists were holding street protests later Tuesday in the capital of Minsk to urge the government to take more vigorous action to tackle the aftermath of the disaster.

At midnight on Monday, a Chernobyl 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 they brought flowers and candles to a memorial for the workers killed in the explosion. Some of the former liquidators dressed in white robes and caps for the memorial, just like the ones they had worn so many years ago.

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 in his mind.

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

In Russia, President Vladimir Putin, in a message to the liquidators, called the Chernobyl disaster "a grave lesson for all of mankind."