Ukraine and other former Soviet republics affected by the Chernobyl nuclear disaster are holding ceremonies on Sunday to mark the 23rd anniversary of the explosion.

Ukrainian President Viktor Yushchenko attended on the night of April 25-26 a church service and took part in a ceremony to lay flowers at the memorial to Chernobyl Heroes in Kiev.

The 1986 Chernobyl disaster is the world''s worst nuclear accident. The catastrophe occurred when reactor No. 4 at the Chernobyl nuclear power plant exploded following a test of the reactor.

Vast areas, mainly in the three then-Soviet republics of Ukraine, Belarus and Russia, were contaminated by the fallout from the critical nuclear meltdown.

Fifty-six people were reported to have been killed at the scene of the disaster, and another 4,000 believed to have died of thyroid cancer shortly afterwards. Several million more people are thought to have been exposed to different degrees of radiation.

About 5 million people still live in areas of Belarus, Russia and Ukraine classified as "contaminated" by radioactive elements.