Under a thin rain, men and women laid wreaths at the foot of a monument to the firemen who died of radiation poisoning after they were sent to clean up the site of the disaster.
"Each year, there are fewer of us to attend this service," said 40-year-old Tetyana Lazarenko, who, along with her family, was evacuated from the town of Pripyat, where Chernobyl employees used to live next to the nuclear power plant, 36 hours after its fourth reactor exploded in April 1986.
"I lost a town, friends, people who were close to me. We all had health problems because of radiation," she added.
"You cannot forget such a tragedy," said Lazarenko, who now lives in Kiev with her husband and three children.
Another overnight service was held at Slavutich, a town in northern Ukraine housing employees who worked at Chernobyl until it was closed down in December
A radioactive cloud was spewed high into the atmosphere when Chernobyl's fourth reactor exploded, burning for 10 days and spreading radioactive material over three-quarters of Europe.
Officially, 31 people were immediately killed by radiation following the blast on April 26, 1986, but unofficial estimates hold that as many as 25,000 of the workers that were sent to clean up the site have since died.
Tens of thousands were crippled from their exposure to high radiation doses and now say their government allowances are not enough to live on.
Over 130,000 people were evacuated from the disaster area and nearly six million continue to live in contaminated zones, in northern Ukraine, as well as stretches of Belarus and Russia.
Ukraine closed down the fourth and last reactor of the Chernobyl power plant in December 2000.