Former workers and relatives of those who died gathered for a candlelight vigil near the plant.
Fire engine sirens went off at the time a reactor at the plant exploded on April 26, 1986. About 30 plant workers and firefighters were killed in the accident.
The blast released massive amounts of radioactive materials and forced more than a 130,000 residents to evacuate the area.
Russia's president Dmitry Medvedev argued Tuesday that tough international guidelines could help prevent accidents like the massive Chernobyl meltdown.
Medvedev and Ukrainian President Viktor Yanukovych took part in a religious service outside Chernobyl's damaged No. 4 nuclear reactor, laying the first stone of a monument to cleanup workers and laying bouquets of red roses at another monument to Chernobyl victims.
Yanukovych stressed that nuclear accidents such as Chernobyl and the nuclear explosion at Japan's Fukushima plant affect the whole planet, renewing calls for money to build a new, safer shelter over the damaged reactor. Ukraine must still raise some $300 million to cover up the plant, which remains a no-go zone a quarter century after the disaster.
The Associated Press contributed to this report.