Ukraine has held a series of events to commemorate the 20th anniversary of the world's worst nuclear disaster at the Chernobyl power plant.
President Viktor Yushchenko joined survivors and relatives of the dead at a ceremony outside the plant.
He met people who worked at the plant and presented medals to those who risked their lives making it safe.
The explosion spewed radioactive fallout over swathes of the then-USSR and many other parts of Europe.
In neighbouring Belarus, also badly affected by fallout, opposition groups held a rally in the capital Minsk to protest against government attempts to rehabilitate contaminated areas.
Nearly one quarter of the country was contaminated by radiation.
Before the rally, authorities sealed off the central October Square - the scene of clashes between opposition supporters and police after the disputed elections won by President Alexander Lukashenko in March.
Thousands of protestors - led by main opposition candidate, Alexander Milinkevich - were forced to gather in a nearby park instead.
The BBC's Emma Simpson in Minsk said opportunities in Belarus for demonstrations were few and protestors used the occasion to accuse authorities of lying about the effects of Chernobyl.
"Chernobyl didn't defeat us and neither will Lukashenko," one party leader said.
The anniversary of the disaster has traditionally been a day of protest for the opposition.
'Ask for forgiveness'
In Ukraine, vigils were held overnight in both the capital, Kiev, and in Slavutych, the town built to house the Chernobyl plant workers displaced by the accident.
In Kiev, hundreds of mourners, each carrying a single carnation and flickering candle, joined an outdoor Orthodox Christian service.
Mr Yushchenko laid a wreath to remember those who were sent to deal with the accident and to the many who have since been affected.
"After 20 years of pain and fear, this land must feel progress," he said.
"The trance we were left in by Chernobyl is over. We are a strong and brave people and we are looking to the future."
At 0123 (2223 GMT on Tuesday) - the precise time an alarm warning of the accident was set off on 26 April 1986 - the church bells tolled 20 times.
A similar ceremony got under way an hour earlier, to coincide with 0123 Moscow time, in Slavutych.
Mourners laid flowers and candles at a monument dedicated to those who died in the immediate aftermath of the accident.
"I knew all of these people," a tearful Mykola Ryabushkin told AFP news agency, pointing to the portraits hanging on the monument.
The 59-year-old had been working as an operator at the plant when the explosion happened.
"I look at them and I want to ask them for forgiveness," he said. "Maybe we're all to blame for letting this accident happen."
Disputed death toll
The accident happened at one of four reactors at the Chernobyl nuclear power plant, 110km (70 miles) north of the capital, Kiev.
Throughout most of the following day the Soviet authorities refused to admit anything out of the ordinary had occurred.
It was only two weeks after the explosion, when radiation releases had tailed off, that the first Soviet official gave a frank account, speaking of the "possibility of a catastrophe".
Official UN figures predicted up to 9,000 Chernobyl-related cancer deaths. A Greenpeace report released last week estimated a figure of 93,000. Greenpeace said other illnesses could bring the toll up to 200,000.
A restricted area with a radius of 30km (19 miles) remains in force around the destroyed nuclear reactor, which is encased in concrete.