Russian president Dmitry Medvedev has called for new safety rules to be drawn up for the global nuclear industry.
The Chernobyl disaster in 1986 has taught states that they must tell the whole truth to their people, Medvedev said, acknowledging the Soviet Union's mistake in delaying announcement of the full scale of the accident for days.
"In order for such tragedies never to be repeated we must all be honest, we must provide absolutely exact information about what is going on," Medvedev said.
The anniversary of Chernobyl has come as Japan struggles to control the crisis at its Fukushima Daiichi nuclear plant hit by the earthquake and tsunami.
Events in Japan and at Chernobyl made it imperative to draw up new rules covering the peaceful use of nuclear energy and safety, Medvedev said.
"I sent proposals to world leaders aimed at guaranteeing the necessary development of nuclear energy in the world while preventing at the same time catastrophic global consequences of accidents."
The No. 4 reactor at the Chernobyl plant exploded and caught fire after a safety test experiment went badly wrong on April 26, 1986, sending radiation across Europe.
A total of 31 people died immediately and many more died of radiation-related sicknesses such as cancer, while tens of thousands were evacuated permanently from Prypyat, the town closest to the site.
The world community, spurred by the nuclear crisis at Japan's Fukushima plant, has pledged 550 million euros (£489 million) to help build a new containment shell over the stricken reactor at the Chernobyl site to replace a makeshift one that has begun to leak radiation.
"Chernobyl was a challenge of planetary dimensions and the answer to this challenge can be provided only by the world community," Ukrainian president Viktor Yanukovich said.
The Chernobyl anniversary has triggered anti-nuclear protests in several European countries, with around 120,000 people taking part in anti-nuclear rallies across Germany the day before.
Chernobyl, seen as the benchmark for nuclear accidents, was relatively untouched in the disaster but Prypyat is now a ghost town at the centre of a largely uninhabited exclusion zone with a radius of 30 km.
Japan has raised the severity rating at Fukishima to seven, the same level as that of Chernobyl.