MOSCOW — As leaders of more than 50 countries prepare to gather in Moscow for a three-day round of summits and celebration marking the 60th anniversary of the end of World War II in Europe, authorities are trying to guarantee two things: sunny skies and no terrorism.

It's unclear which is the bigger challenge.

As for any threatening rain clouds, they are due to be attacked by Russian air-force planes at some distance from Moscow starting today. Chemical agents spread by the planes are intended to trigger rainfall from the clouds before they reach Moscow, where President Bush will join Russian President Vladimir Putin and about 8,000 other guests in Red Square on Monday to watch a military parade — presumably in bright sunshine.

For the battle against terrorist attacks, authorities are mobilizing 30,000 police and severely restricting vehicle traffic in the heart of Moscow. Exit from some downtown subway stations will be allowed only by special pass. Police have been conducting strict document checks in the capital for days.

Russian air-force chief Vladimir Mikhailov acknowledged recently that the cloud-seeding effort would not guarantee fine weather. But he assured reporters there was no chance any unauthorized aircraft could violate the "no-fly" zone routinely in place over Moscow and stage an attack on visiting dignitaries. "Fighter crews will cut short the actions of any intruder aircraft," he said.

The Victory Day parade, involving about 7,000 soldiers and 2,600 World War II veterans, will conclude with 12 planes flying in tight combat formation over Red Square, trailing plumes of white, blue and red smoke to imitate the Russian flag.

The greatest fear is an attack by separatists fighting for independence in Chechnya. Last year, the Russian republic's pro-Moscow president was killed by a bomb blast during a Victory Day parade in the Chechen capital, Grozny.

"The danger of terrorism is very real," said Stanislav Belkovsky, president of the National Strategy Council, a Moscow think tank. "The authorities have every reason to be afraid of Chechen terrorists. ... I think foreign leaders' security services are equally aware of the danger, and they will certainly take their own measures to protect their leaders."

Scheduled events begin tomorrow with a summit of the Commonwealth of Independent States (CIS), a club of a dozen former Soviet nations that has had trouble defining a reason for its existence. Azerbaijani President Ilham Aliyev is boycotting the gathering, having stated that he does not want to sit down with Armenian President Robert Kocharyan. The two countries have a bitter and long-running territorial dispute.

Georgian President Mikheil Saakashvili will boycott both the CIS summit and the parade, Georgian Foreign Minister Salome Zourabichvili said in Moscow yesterday, after negotiators failed to reach agreement on a timetable for withdrawal of Russian military bases from Georgia. Putin said this year that the CIS had served a valuable purpose by easing the trauma of the 1991 collapse of the Soviet Union and that it should not be seen as a vehicle to build great unity among member states.

The CIS heads of state are expected to discuss collective efforts against terrorism, the boosting of cultural and educational ties, and possible reform of the group's structure.

Bush and Putin have scheduled a summit and dinner tomorrow. Bush wants Putin's help in dealing with international issues such as the spread of nuclear weapons but also is committed to pressing for greater democracy in Russia.

As part of his balancing act, Bush is tentatively scheduled to meet Monday with pro-democracy representatives of Russian civil society, in something of a throwback to the days when U.S. presidents met on the side with Soviet dissidents during visits to Moscow.

Putin is likely to press Bush for assurances that the American effort to promote democracy around the world will not extend to an attempt to destabilize his government, Belkovsky said.

British Prime Minister Tony Blair telephoned Putin yesterday to say that he could not attend Monday's parade, the Kremlin news service said. Blair will be busy forming a new government after parliamentary elections.

At a summit Tuesday between Russia and the European Union, the two sides are expected to adopt plans to increase cooperation in four broad fields: economics and trade; freedom and justice; external security; and scientific research, education and culture.