KIEV, April 26 - President Viktor Yushchenko and dozens of mourners prayed and lit candles before dawn Thursday to mark the precise time the Chornobyl nuclear reactor exploded and caught fire 21 years ago in what became the world's worst nuclear accident.

Chornobyl's Reactor No. 4 exploded at 1:23 a.m. on April 26, 1986, spewing radiation over a large area of the former Soviet Union and much of northern Europe.