Hundreds of mourners laid flowers and lit candles early Tuesday before a monument in Ukraine's (search) capital to mark the 19th anniversary of the Chernobyl (search) nuclear disaster, which spewed radiation over much of northern Europe and claimed thousands of lives.

As the country slept on April 26, 1986, a test at the then-Soviet Chernobyl nuclear power station went horribly wrong, causing Reactor No. 4 to explode and catch fire.

"The Chernobyl plant that was regarded as Ukraine's pride has become a symbol of the biggest ever man-made disaster," the plant's management said in a statement Tuesday, a day that is now observed worldwide as a memorial to victims of radiation catastrophes.

An area roughly half the size of the U.S. state of Colorado was contaminated by the accident, forcing the resettlement of hundreds of thousands of people and ruining some of Europe's most fertile farmland.

Ukraine has registered 4,400 deaths. In all, 7 million people in the former Soviet republic's of Belarus, Russia and Ukraine are believed to have suffered health problems. Many were the firefighters, cleanup workers, soldiers and scientists sent in to help deal with the tragedy.

"They protected us like heroes of war," said Ganna Romanova, 75, a survivor of the disaster. "We must not forget them and we must tell our children about their feat."

Chernobyl was shut down in 2000, but decommissioning work continues.