By Helen Fawkes
BBC News, Kiev
At 1.23am in Kiev, a single bell tolled 20 times.
Candles are lit to remember those who died as a result of Chernobyl
An outdoor memorial service was being held at the exact moment that one of the reactors at the Chernobyl nuclear plant exploded two decades ago.
Hundreds of people gathered for the Orthodox Christian ceremony in the Ukrainian capital.
It is organised every year in the grounds of a small white church dedicated to the Chernobyl accident.
Candles flicker against the night sky and the smell of incense hangs in the air. Some of the women quietly cry as a minute's silence is observed.
It is a solemn moment to reflect the world's worst nuclear disaster, which happened here in Ukraine.
Ekaterina says she remembers the sunny day back in 1986 when tragedy struck.
"I was crossing the road and all of a sudden I saw a policeman wearing full protective gear and carrying a gun. I was just stunned," says the retired school teacher who lived nearby.
"I know lots of widows now because of the accident. So many lives were destroyed by Chernobyl. It's a terrible waste."
At the time of the nuclear catastrophe, Chernobyl was located in the Soviet Union.
There is still anger about the way people were treated by the authorities.
The evacuation did not start for more than a day after the initial blast.
"We were told we were going to be leaving our homes for three days that was 20 years ago," says Olga, who lived in a town built for Chernobyl workers, which now stands deserted.
You cannot see or smell radiation.
But step inside the 30km exclusion zone around the nuclear power plant and you can hear it. In some parts, Geiger counters which are used to check radiation levels screech as they go off the scale.
The zone is similar to the size of Greater London. It is an eerie place, now virtually empty.
Some families still farm inside the 30km exclusion zone
Nature is reclaiming the buildings abandoned by the mass evacuation.
A few hundred people, like Masha Upora, have come back illegally.
The pensioner lives off the contaminated land, she has a cow and a few chickens. Masha takes the risk because for her, this is home.
"The authorities tell us that it's dangerous here and we should leave. But I'm not afraid of radiation because I can't feel it."
But millions have been affected by the disaster.
The explosion and fire which ripped through the nuclear reactor released a huge radioactive cloud over Europe.
To remember those people, Ukraine's President Viktor Yushchenko laid a bunch of red roses at the Chernobyl moment. He was accompanied by the prime minister and the parliamentary speaker.
The flowers were placed on stone stabs which bear the names of some of those who died.
The estimates of how many people Chernobyl will eventually kill range from a few thousand to almost 100,000.
"A good friend of mine who was only in his thirties died of cancer, we believe it was because of Chernobyl. I was deeply shocked by his death," said former world heavyweight boxing champion Vitaly Klitschko, as he left the early morning memorial service.
"The disaster changed Ukraine for ever. A tragedy like this should never be allowed to happen again."
There is a mood of reflection as events are held across the country to mark the anniversary.
Many here are keen for Ukraine to move out of the shadow of Chernobyl, but at the same time they also hope the world will never forget what happened here two decades ago.