Russia's president argued Tuesday that tough international guidelines could help prevent accidents like the massive Chernobyl meltdown, defending nuclear energy during solemn ceremonies commemorating the 25th anniversary of the worst nuclear accident in history.

Dmitry Medvedev and Ukrainian President Viktor Yanukovych took part in a religious service outside Chernobyl's damaged No. 4 nuclear reactor, laying the first stone of a monument to cleanup workers and laying bouquets of red roses at another monument to Chernobyl victims.

Medvedev said he has invited world leaders to work on rules for safer nuclear energy. His economic adviser, Arkady Dvorkovich, said Russia forwarded its proposals to leaders of other Group of Eight countries Tuesday, and he hoped they would be discussed at next month's summit in France.

"It's of utmost importance that we understand what kind of force humankind is dealing with so that our solutions ... meet the challenges of nuclear energy," Medvedev said.

The accident on April 26, 1986 spewed a cloud of radioactive fallout over much of Europe and forced hundreds of thousands from their homes in heavily hit areas of Ukraine, Belarus and western Russia. It has left forests and farmland still contaminated, offering a warning to the Japanese of the potential long-term effects of their own nuclear disaster at the Fukushima Dai-ichi nuclear plant.

The accident fostered deep mistrust among many in the affected areas, where Soviet leaders waited for days to inform people of the accident, to evacuate them from contaminated areas, and to warn them how to reduce health risks. Medvedev called that a major mistake.

"The duty of the government is to tell its people the truth. We must admit that the government did not always behave in the right way," he said. "We must all be honest, we must give absolutely clear information about what is going on."

Yanukovych stressed that nuclear accidents such as Chernobyl and the nuclear explosion at Fukushima affect the whole planet, renewing calls for money to build a new, safer shelter over the damaged reactor. Ukraine must still raise some $300 million to cover up the plant, which remains a no-go zone a quarter century after the disaster.

"The whole world has become convinced that such catastrophes have no boundaries and Fukushima-1 serves as a bitter example of that," Yanukovych said. "No nation can battle such catastrophes alone."

Despite the dangers, the three most affected former Soviet countries continue to believe in nuclear energy. Vladislav Bochkov, spokesman for the Russian nuclear energy agency, said 11 reactors are now under construction in Russia. Ukraine is building two and Belarus is building one reactor.

The reactor in Belarus is being constructed close to the border with Lithuania, where protests were held Tuesday by activists who believe the project is unsafe.

The Kremlin said Medvedev is calling for stricter safety standards for building and operating nuclear power plants, increasing governments' responsibility when dealing with the consequences of possible nuclear accidents and obliging governments to provide full information on possible nuclear disasters.

The Chernobyl explosion released about 400 times more radiation than the U.S. atomic bomb dropped over Hiroshima. The U.N.'s World Health Organization said at a conference in Kiev last week that among the 600,000 people most heavily exposed to the radiation, 4,000 more cancer deaths than average are expected to be eventually found.

Artur Tverdokhlebov, 80, a retired subway worker, joined some 3,000 Chernobyl victims at a memorial service at a monument in Kiev.

"Chernobyl is an open wound in the soul of our people," said Tverdokhlebov, who was rushed to clean up the aftermath of Chernobyl in May 1986. "The authorities kept secret what had really happened, nobody told us anything about the danger and we ate the fish that we caught in the river."

Russia, Ukraine and Belarus have cut the benefits packages for sickened cleanup workers in recent years and the memorial events were overshadowed by their complaints for more aid. Prime Minister Mykola Azarov vowed Tuesday that benefits to Chernobyl victims would continue to be paid.

Belarusian President Alexander Lukashenko, who has been blacklisted by the European Union after a violent crackdown on opposition activists late last year, did not take part in memorial events in Ukraine.

He suggested that he had not been invited.

"Ask Yanukovych that question — why isn't the Belarusian president present at their events? Ask them that," Lukashenko told reporters on a visit to Chernobyl-contaminated regions in Belarus. "Unfortunately, the current Ukrainian leadership is really lousy."

Some observers believe that Ukraine wanted to mark the Chernobyl anniversary without Lukashenko to please Brussels as it seeks EU membership.

The European Commission last week pledged another euro110 million ($156 million) to programs to liquidate the consequences of the Chernobyl explosion.

Lukashenko said Belarus was also in need of Western help but had no intention of asking.

In past years on the Chernobyl anniversary, the Belarusian opposition has led a protest march through the capital, Minsk, channeling anger toward the authoritarian government and fears that it is hiding the truth about the consequences of the nuclear disaster.

This year, the march was banned and an evening rally relegated to a park on the outskirts.


Anna Melnichuk in Kiev, Nataliya Vasilyeva in Moscow and Yuras Karmanau in Minsk contributed to this report.