Kiev, Ukraine ? Ukrainian mourners carried single red carnations and flickering candles during a solemn ceremony early today to remember the 1986 Chernobyl explosion, an event that continues to scar this ex-Soviet republic 20 years later.

The April 26, 1986, pre-dawn explosion became the world’s worst-ever nuclear accident, ripping off the nuclear power plant’s roof and spewing radioactive fallout for 10 days over 77,220 square miles of the then-Soviet Union and Europe. It cast a radioactive shadow over the health of millions of people; many believe it also contributed to the eventual collapse of the Soviet Union.

“My friends were dying under my eyes,” said Konstantyn Sokolov, a 68-year-old former Chernobyl worker whose voice was hoarse from throat and lip cancer. Sokolov was among hundreds gathering for a middle of the night ceremony today in the Ukrainian capital, which President Viktor Yushchenko attended.

In Kiev, bells tolled 20 times starting at 1:23 a.m., marking the time of the explosion at Reactor No. 4 at the Chernobyl nuclear power station. Orthodox priests led the mourners in a somber procession.

Closer to Chernobyl in Slavutych – the town built to house Chernobyl workers displaced in the accident – the commemorations began an hour earlier to coincide with Moscow time, which was used in the then-Soviet Republic of Ukraine at the time of the accident.

Residents laid flowers and placed candles at a monument dedicated to Chernobyl as sirens blared.

Death tolls connected to the blast remain hotly debated, as do the long-term health effects.

Chernobyl veteran Oleksiy Kashcheyev, lights a candle in a church built to commemorate victims of the worst ever nuclear disaster, during a night ceremony on the occasion of the 20th anniversary of the explosion in Reactor No.4 at the Chernobyl nuclear power plant, early Wednesday, April 26, 2006, in Kiev, Ukraine.

At least 31 people died as a direct result of trying to keep the fire from spreading to the plant’s three other operating reactors. One plant worker was killed instantly and his body has never been recovered. Twenty-nine rescuers, firefighters and plant workers died later from radiation poisoning and burns, and another person died of an apparent heart attack.

Thousands have been diagnosed with thyroid cancer, one of the only internationally accepted illnesses linked to Chernobyl, and the U.N. health agency said about 9,300 people were likely to die of cancers caused by radiation.

Some groups, however, including Greenpeace, have warned that death tolls could be 10 times higher and accused the U.N. of whitewashing the long-term effects of the accident in order to restore trust in the safety of atomic power.

Around 350,000 people were evacuated forever from their homes, leaving the whole city of Pripyat and dozens of villages to decay and rot away. Experts say some may not be habitable again for centuries.