Hundreds of survivors gathered at monuments to those who died cleaning up after the disaster, holding flowers and candles at overnight ceremonies in Ukraine where the plant is located.
A minute's silence was held in Ukraine to mark the two blasts which ripped through reactor number four at 1.23am on 26 April 1986.
Around five million people are believed to have been affected by the disaster in Belarus, Russia and Ukraine, all of which still have regions where the levels of radiation are much higher than accepted norms.
Officials estimates suggest between 4,000 and 9,000 people could be expected to die overall as a direct consequence of the accident, although environmental groups have put the figure at 100,000 and higher.
Mourners and others affected by the disaster gathered at a memorial in the northern Ukrainian town of Slavutich, built to house workers of the station after the accident.
"I lived through all this," said 59-year-old Mykola Ryabushkin, wiping away streaming tears.
Mr Ryabushkin was an operator on duty at the Soviet nuclear power plant when the explosions shook the station, bathing it in an eerie bluish light and spewing huge amounts of radiation into the air.
"I knew all of these people," he said, pointing to photographs of 30 people who died in the first year after the disaster which hung on a memorial in Slavutich's central square.
"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."
Hundreds of the sombre-faced people who clutched candles and carnations at the service had similar stories.
The town lies 50 kilometres east of Chernobyl and rose out of the ashes of the disaster to house many of those who were evacuated after the accident.
Galina was a 25-year-old engineer also working on the night of the blasts. She came close to being one of the first batch of victims.
"Since it was my birthday (on the 26th), I was allowed to go home early, a half hour before the blast," she said. "This saved my life."
Ukrainian President Viktor Yushchenko led another 300 people who attended a memorial service at a church in eastern Kiev that features memorial plaques of many of those who were involved in the immediate aftermath.
The cloud released by the Chernobyl explosion settled mostly in Ukraine and neighbouring Belarus to the north, but parts of it drifted across Russia and a large swathe of Europe, and its effects were felt from Scandinavia to Greece.
The impact was made worse by the fact that the then Soviet authorities concealed the extent of what had happened for several days and did not begin to evacuate people from the area until more than a day and half later.
The anniversary is also a highly charged one for Belarus, whose territory lay in the immediate path of the radioactive cloud.
In the capital Minsk, opposition groups held an evening demonstration against the government's handling of Chernobyl's aftermath, a demonstration that has become an annual event known as the "Chernobyl Way".
Work on a new protective covering around the plant to replace the concrete sarcophagus that was hastily constructed immediately after the accident is due to begin this summer and is expected to be finished in 2010.
The 20,000-ton steel case is expected to cost between one and two billion dollars.