The meltdown is considered the world's worst nuclear disaster.
Bells and sirens throughout Ukraine sounded Tuesday at 1:23:58 a.m. — the moment the plant's reactor No. 4 exploded on April 26, 1986.
In the city of Slavutych — built for workers evacuated from the Chernobyl plant in the former Soviet Union — a remembrance ceremony was held for the "liquidators," the term for the thousands of military personnel and volunteers who responded to the unfolding accident without suitable protective equipment.
Ukraine President Petro Poroshenko led a ceremony in Chernobyl, where work is underway to build a new cover for the reactor, due to be completed in 2017.
Many liquidators have since died or are ill from radiation.
“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.”
In Minsk, the capital of Belarus, where land was affected by the Chernobyl fallout, more than 1,000 people held a protest march through the city center, the Associated Press reported.
Belarus usually cracks down on dissent but allowed the march.
Russian President Vladimir Putin sent a message Tuesday to the liquidators.
"Chernobyl has become a serious lesson for all mankind, and to this day it has severe repercussions on both the environment and human health. " Putin said.
"The scale of the tragedy could be immeasurably greater, if it were not for the unprecedented courage and dedication of the firefighters, military personnel, experts, medical workers who honorably fulfilled their professional and civic duty. Many of them sacrificed their own lives to save others."
Relatives of those who died attended candlelight vigils, and flowers were placed at memorials and churches in Ukraine's capital of Kiev and elsewhere.
The international community on Monday pledged an extra $100 million to Ukraine to help build a new underground storage facility for hazardous nuclear waste at the plant.
More than $2.2 billion has already been donated to help Ukraine build the new reactor cover.
Source: USA Today