Hundreds of people, each bearing a candle and some with red carnations, filed slowly through the streets of Slavutych, the town built to house the plant's workers after the accident on April 26, 1986.
A minute of silence was declared at 1.23am Moscow time (2123 GMT) - about the time of the explosion and the subsequent fire that sent radiation billowing throughout Europe.
A bell tolled and alarm sirens blared.
A middle-aged man, tears welling in his eyes, shook his head in disbelief as he stood alongside younger mourners.
Well after midnight, in Kiev, 80km to the south, Viktor Yushchenko, the Ukrainian president, stood alongside other dignitaries by the "Chernobyl church", where survivors gather every year, and placed a large bouquet of roses at a memorial marked by two stone slabs on a knoll.
The memorial bears the names of the "liquidators", the firefighters and engineers who died trying to extinguish the blaze, or later from excessive doses of radiation.
The blast in Chernobyl's fourth reactor - during an unexplained experiment - contaminated large swaths of territory in Ukraine, Belarus and Russia.
Chernobyl was the world's worst
Soviet authorities took two days to inform the world and their own people. They then launched feverish clean-up and reconstruction efforts culminating in construction of a shelter to house the wrecked reactor.
The Slavutych procession moved to a memorial, with mourners placing candles at the foot of a wall bearing images graved in stone of the "liquidators".
Varying death tolls
Estimates of the toll linked to Chernobyl vary widely.
Radiation released estimated at equivalent of 400 Hiroshima bombs
29 rescuers, firefighters and plant workers died from radiation in the immediate aftermath of explosion
350,000 people were evacuated from the contaminated zone around reactor
UN agencies estimate 4,000-9,000 will die as a result of the disaster
Environmental groups say toll will reach 100,000
The World Health Organisation (WHO) puts at 9000 the number of people expected to die due to radiation exposure, while the environmental group Greenpeace predicts an eventual toll of 93,000.
Hundreds of thousands were evacuated. The United Nations says 7 million still live on land with unsafe radiation levels.
Yushchenko, due to visit the 30km (19 mile) "exclusion zone" around Chernobyl later in the day, appealed before the anniversary for financial help to build a new "sarcophagus" to replace the leaking original containment structure.
International figures said the main lesson was to adopt a common approach to nuclear safety.
Mohamed ElBaradei, head of the International Atomic Energy Agency, (IAEA), which helped to investigate the accident, called for closer co-operation, "particularly at a time when we are witnessing an expansion of nuclear power to meet increasing energy demands in many parts of the world".
George Bush, the US president, paid tribute to "lives lost and communities hurt in the devastation" and said Washington was committed to efforts "to improve the safety and security of Chernobyl by confining its nuclear reactor".
The 'tomb' project
Vladimir Putin of Russia, decorating "liquidators" in Moscow, promised to look into setting up a treatment centre for people suffering the ill-effects of exposure to large doses of radiation.
Chernobyl clean up is to cost
$800 million to $1.4 billion
"These people who worked there did not think of themselves, they understood that the disaster had to be stopped, whatever the cost," Putin said.
Yushchenko told officials assessing the effects of Chernobyl on Monday that a new conference of donors was needed to complete the "tomb" project launched in the 1990s.
Ukraine, which spent up to 10% of its budget on the post-Chernobyl cleanup, cannot afford to take on the project alone. It has a price tag of $800 million to $1.4 billion.
Experts see the construction of a new "sarcophagus" as part of a plan to decommission the station, which stopped producing electricity in 2000 at the insistence of the international community, but still contains 200 tonnes of nuclear fuel.