CHERNOBYL, Ukraine - Ukraine marked the 25th anniversary on Tuesday of the world's worst nuclear accident at its Chernobyl power plant as Japan pressed on with efforts to control the crisis at its Fukushima plant.

On April 26, 1986, the No 4 reactor at the Chernobyl plant, then in the Soviet Union, exploded and caught fire after a safety test experiment went badly wrong.

The blast sent radiation billowing across Europe.

A total of 31 people died immediately but many more died of radiation-related sicknesses such as cancer, many of them in what is today Belarus.

Tens of thousands were evacuated, never to return, from Prypyat, the town closest to the site which then had a population of 50,000.

Last week the world community, spurred by the nuclear crisis at Japan's Fukushima plant, pledged 550 million euros ($780 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. The answer to this challenge can be provided only by the world community," Ukrainian President Viktor Yanukovich said on Tuesday.

"For a long time, Ukraine was alone with this calamity, but happily we are not alone now," he said in a statement on the presidential website president.gov.ua.

Yanukovich was to visit Chernobyl later on Tuesday with Russian President Dmitry Medvedev and Russian Orthodox Patriarch Kirill.

Chernobyl has remained the benchmark for nuclear accidents.

Though Chernobyl town itself was relatively untouched by the accident, Prypyat is now a ghost town at the center of a largely uninhabited exclusion zone with a radius of 30 kilometers.

On April 12 Japan raised the nuclear severity rating at its Fukushima plant to seven, the same level as that of Chernobyl.

"This is a day of mourning for us. We are in mourning for the people who 25 years ago fought to protect us," said Gennady Pikul, 50, referring to firefighters and other "liquidators" who at risk to their lives fought to control the blazing reactor.

"We will do everything we can so that this is never repeated," he said.

The 550 million euros raised at the donors' conference last week will be added to cash already contributed for construction of a new 110-meter-high encasement over the reactor and a storage facility for spent fuel.

Soviet officials in 1986 withheld reporting the accident for two days, provoking accusations that they were trying to cover up the accident.

Medvedev, meeting survivors of Chernobyl clean-up efforts in the Kremlin on Monday, said there must be greater transparency in nuclear emergencies.

"I think that our state must learn the lessons from what happened from the now-distant Chernobyl incident in 1986 and the recent tragedies in Japan. Perhaps the most important lesson is the need to tell people the truth," he said.

Reuters