Many who lost relatives in the Chernobyl nuclear disaster, like Praskoviya Nezhyvova whose son Viktor died, have been marking the 20th anniversary.
At the memorial in Ukraine, a man who worked at Chernobyl drinks to the memory of friends and colleagues lost in the disaster.
Hundreds of Ukrainians gathered in the early hours to mark the anniversary.
Mourners carried carnations and candles to remember the victims of the world's worst nuclear accident.
President Viktor Yushchenko was among those who gathered for a service at the "Chernobyl church" in the capital Kiev.
Similar ceremonies were held in Slavutych, the town built to house Chernobyl workers displaced by the accident.
Their ceremonies began an hour earlier to coincide with Moscow time, which was used in the then-Soviet Republic of Ukraine.
Alarm sirens blared and bells tolled as residents laid flowers and candles at a monument dedicated to those who died.
In the Russian capital, Moscow, a disabled man who helped in the Chernobyl rescue operation visits another memorial.
The disaster was also marked in other capitals. In Madrid, Spain, protesters from environmental group, Greenpeace, laid down in a square to commemorate the accident's victims.