Russian President Dmitry Medvedev speaks during the awarding ceremony for veterans of the Chernobyl nuclear disaster cleanup operation in Moscow's Kremlin, Monday, April 25, 2011. The main lesson of the nuclear disasters at Chernobyl in 1986 and Fukushima last month is that authorities must tell the truth about the situation, Medvedev said on April 25. (AP Photo/Dmitry Kostyukov, Pool)

Russian President 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 placing 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 Tuesday to leaders of other Group of Eight countries, 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 Chernobyl accident fostered deep mistrust among many in the affected areas, because Soviet leaders waited for days to tell people about the accident, evacuate them from contaminated areas and 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." The Kremlin said Medvedev was calling for stricter safety standards for building and operating nuclear power plants, and increased governmental responsibilities when dealing with the consequences of possible nuclear accidents, including providing full, accurate information on any accident.

Russia's President Dmitry Medvedev, right, speaks with a veteran of the Chernobyl "liquidators", emergency workers who fought the blaze at the Chernobyl nuclear reactor, on the eve of the 25th anniversary of the disaster in Moscow's Kremlin, Monday, April 25, 2011. (AP Photo/RIA-Novosti, Dmitry Astakhov, Presidential Press Service)

"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 new reactor being constructed in Belarus is close to the border with Lithuania, where protests were held Tuesday by activists who believe the project is unsafe. Opposition activists in Belarus also rallied to protest the new reactor.

U.N. Secretary-General Ban Ki-moon, who visited Chernobyl last week, stressed in a statement Tuesday the importance of strengthening the global nuclear safety regime.

The Chernobyl explosion released about 400 times more radiation than the U.S. atomic bomb dropped over Hiroshima. The U.N. World Health Organization said among the 600,000 people most heavily exposed to radiation at Chernobyl, 4,000 more cancer deaths than average are expected.

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."

In Moscow, hundreds gathered at a cemetery where 28 Chernobyl firefighters are buried.

Russia, Ukraine and Belarus have reduced the benefits packages for sickened cleanup workers in recent years and the memorial events were overshadowed by their complaints for more aid.

Belarusian President Alexander Lukashenko, who has been blacklisted by the European Union after a violent crackdown on protesters alleging voting fraud last year, did not take part in the memorials 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.

Russia's President Dmitry Medvedev poses for a photo with Vyacheslav Gryzunov, right, during a ceremony to award "liquidators", emergency workers who fought the blaze at the Chernobyl nuclear reactor, on the eve of the 25th anniversary of the disaster in Moscow's Kremlin, Monday, April 25, 2011. (AP Photo/Alexander Natruskin, Pool)

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 Lukashenko's 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 to protest the construction of Belarus' new nuclear power plant was relegated to a park on the capital's outskirts.

Several hundred activists took part, and each of them had to pass a security check and be photographed by security agents before being allowed to join the demonstration.

They held posters saying, "Yesterday Chernobyl, today Fukushima, tomorrow the Ostrovetskaya nuclear power plant. No thank you!"