A quarter of century ago, workers at the Chernobyl atomic power station were carrying out a test on a reactor when operating errors and design flaws sparked successive explosions. The accident caused catastrophic damage to both human life and the environment.

On the 25th anniversary of the Chernobyl nuclear disaster, Ukraine has held somber ceremonies to commemorate the special date.

The ceremonies were held in the capital Kiev, with hundreds of people gathering at a church memorial. The Russian Orthodox patriarch commemorated the victims with prayers and candle-lighting.

Later Tuesday, the presidents of Ukraine and Russia, Viktor Yanukovich and Dmitry Medvedev, are expected to visit the stricken plant. It's still surrounded by a 30-kilometer exclusion zone. On April 26th, 1986, there was a huge blast at the Chernobyl nuclear plant's Number 4 reactor.

Two workers were killed immediately and 28 other rescuers and staff died of radiation exposure in the following months. The explosion spewed a cloud of radioactive fallout over Belarus, Russia and western Europe. Hundreds of thousands were evacuated from their homes. Fears still remain today of the scale of damage to people's health.