Russian President Vladimir Putin on Tuesday told the annual Victory Day parade on Red Square that the horrors of World War II demonstrate the necessity of countries working together to prevent war.

Russia celebrates the 1945 defeat of Nazi Germany every May 9 to honor the 8 million Red Army soldiers who fought and died for their country.

"This monstrous tragedy was not able to be prevented primarily because of the connivance of the criminal ideology of racial superiority and due to the lack of unity among the world's leading nations," Putin said.

"To effectively combat terrorism, extremism, neo-Nazism and other threats, consolidation of the entire international community is necessary," he said.

Russian servicewomen march at Red Square during the Victory Day military parade in Moscow (Getty Images)

Russian servicemen march at Red Square during the Victory Day military parade in Moscow (Getty Images)

(Reuters)

(Reuters (Reuters)

The Soviet Union is estimated to have lost 26 million people in the war, including the 8 million soldiers. The nation's immense suffering contributes to Victory Day's status as Russia's most important secular holiday.

Thick clouds over Moscow on Tuesday, however, forced the cancellation of the traditional dramatic conclusion to the parade — the roaring flyover by scores of military aircraft.

The Red Square parade is a highly ritualized display, and marked changes in its order are unusual.

The Defense Ministry had said cloud-seeding planes would be deployed to disperse the overcast skies. That has been done previously when poor weather threatened. It wasn't immediately clear if the planes had been deployed.

Parade marking the World War II anniversary at Red Square in Moscow (Reuters)

Female servicewomen march during a Victory Day military parade marking the 72nd anniversary of the victory over Nazi Germany in the Great Patriotic War, the Eastern Front of World War II (Getty Images) (Lev Fedoseyev\TASS via Getty Images)

Russia's President Vladimir Putin and Russia's Defence Minister Sergei Shoigu in Moscow's Red Square after a Victory Day military parade (Getty) (Mikhail Klimentyev\TASS via Getty Images)

Parades also were held across Russia's sprawling expanse as well as in the Russia-annexed Crimea Peninsula, but the Red Square procession is the centerpiece of Russia's observances.

About 10,000 soldiers participated, standing rigidly as Defense Minister Sergei Shoigu reviewed them from an open-top limousine. The soldiers then marched out to make way for a display of military vehicles ranging from armored cars to lumbering Topol intercontinental ballistic missile launchers.

The parade gave the first public showing of the Tor and Pantsir mobile surface-to-air missiles that have been adapted for use in Russia's Arctic forces, their white-and-black winter camouflage standing out amid the olive drab of other war machines.

"The armed forces of Russia are capable of repelling any potential aggression," Putin said.

Russian female army soldiers march along the Red Square during the Victory Day military parade to celebrate 72 years since the end of WWII and the defeat of Nazi Germany, in Moscow, (AP)

S-400 Triumf surface-to-air launch vehicles seen during the parade (Getty Images) (Viktor Drachev\TASS via Getty Images)

In the afternoon, hundreds of thousands of people braved temperatures near freezing to march in the "Immortal Regiment" demonstration that honors those who fought in the war.

The throng, many of them bearing photos of relatives who endured the war, covered a six-kilometer (3} -mile) route down Tverskaya Street, Moscow's most iconic avenue, and through Red Square. It was a show of determination to keep the war's renown alive as living memory of it dwindles

Interior Ministry spokeswoman Irina Volk said the crowd numbered about 850,000.