The accident spread radioactive fallout across Europe in 1986, particularly contaminating Ukraine, Russia and Belarus.
"Chernobyl will always remain an open wound in the heart of our country, in the hearts of millions of people," President Petro Poroshenko wrote on Facebook.
Several hundred people gathered overnight for a ceremony at a memorial to victims in Slavutych, a town 50 kilometres (30 miles) from the nuclear power plant.
Mourners lit candles and put flowers in front of memorial plaques. Many of the power station's personnel moved to Slavutych after the accident.
In Minsk, people also gathered at a memorial to victims and soldiers marched bearing baskets of flowers.
On April 26, 1986, reactor number four at the Chernobyl plant, some 100 kilometres north of the capital Kiev, exploded during a botched safety test.
The reactor burned for 10 days, sending radioactive elements into the atmosphere that contaminated three-quarters of Europe, according to some estimates.
Soviet authorities tried to cover up the accident. Sweden was the first to raise the alarm after scientists detected a spike in radiation levels on April 28. Soviet leader Mikhail Gorbachev did not make a public statement until May 14.
A total of nearly 350,000 people living within a radius of 30 kilometres of the plant were evacuated for many years.
About 600,000 Soviet citizens who became known as "liquidators" -- mostly emergency workers and state employees -- were dispatched with little or no protective gear to help clean up and build a concrete sarcophagus over the damaged reactor.
The number of deaths directly due to the accident is still a matter of debate, with estimates varying from around 30 to one hundred thousand.
In November 2016, a massive metal dome was erected over the remains of the reactor paid for with 2.1 billion euros ($2.5 billion) in international funding to stop future leaks and ensure the safety of Europeans for generations.
As a result, radiation levels close to the reactor fell by 90 percent over the course of a year, according to official estimates.
France24
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