The world on Tuesday, April 26, marked a quarter century since the worst nuclear accident in history at Chernobyl in Ukraine, haunted by fears over the safety of atomic energy after the Japan earthquake.

Russian President Dmitry Medvedev is to make a landmark visit to Chernobyl later the day to take part in memorial ceremonies at the site, joined by his Ukrainian counterpart Viktor Yanukovych, according to AFP.

The commemorations began with a service led by Russian Orthodox Patriarch Kirill at a monument dedicated to Chernobyl clean-up workers in the Ukrainian capital Kiev. A bell struck at 1:23 am (2223 GMT Monday) - the moment when the explosion happened - and tolled 25 times for the number of years that have passed since the disaster.

In the early hours of April 26, 1986, workers at the Chernobyl atomic power station in the then Soviet republic were carrying out a test on reactor four when operating errors and design flaws sparked successive explosions.

Radioactive debris landed around the reactor, creating an apocalyptic scene in the surrounding area, while material also blew into the neighboring Soviet republics of Belarus and Russia and further into western Europe.

Two workers were killed by the explosion and 28 other rescuers and staff died of radiation exposure in the next months. Tens of thousands needed to be evacuated and fears remain of the scale of damage to people's health.

The anniversary has gained an eerily contemporary resonance after the March 11 earthquake and tsunami in Japan which damaged reactors at the Fukushima Daiichi power plant and prompted leaks of radiation.