President Vladimir Putin, the longest-serving Kremlin chief since Josef Stalin, stood beside China's Xi Jinping, several dozen other leaders and Russian veterans on a roofed tribune beside Lenin's mausoleum as Russian troops marched past.

Putin said Russia would never accept attempts to belittle the Soviet Union's decisive role in defeating Nazi Germany, but that Moscow also recognised the part played by the Western allies in defeating Adolf Hitler.

"The Soviet Union took upon itself the most ferocious, merciless blows of the enemy," Putin said.

"We highly appreciate the contribution of the soldiers of the Allied armies, the members of the resistance, the courageous people of China, and all those who fought for a peaceful future to our common struggle."

Putin made no criticism of the West and referred only in passing to the Ukraine war, Europe's deadliest since World War Two, but it haunted the celebration. 

More than 11,500 troops were lined up in ranks on Red Square, including 1,500 who have fought in Ukraine. Drones - the biggest technological innovation of the war - were paraded for the first time, as well as tanks and intercontinental Yars missiles capable of carrying nuclear warheads. 

Ukraine attacked Moscow with drones for several days this week, though there were no reports of major attacks on Moscow on Friday amid a 72-hour ceasefire declared by Putin.

The Soviet Union lost 27 million people in World War Two, including many millions in Ukraine, but pushed Nazi forces back to Berlin, where Hitler committed suicide and the red Soviet Victory Banner was raised over the Reichstag in 1945.

Chinese Communist Party historians say China's casualties in the 1937-1945 Second Sino-Japanese War were 35 million. The Japanese occupation caused the displacement of as many as 100 million Chinese people and significant economic hardship, as well as the 1937 Nanjing Massacre, during which an estimated 100,000 to 300,000 victims were killed.

Moscow and Kyiv do not publish accurate casualty numbers for the war in Ukraine, though US President Donald Trump, who says he wants peace, says hundreds of thousands of soldiers on both sides have been killed and injured.

More than two dozen foreign leaders are due in Moscow on Friday for Russia's flagship Victory Day military parade.

The two most important guests at the parade are China's Xi and Brazil's Luiz Inacio Lula da Silva.

Xi is being treated to a state visit in Moscow and on Thursday held a series of talks with Putin on deepening the allies' "no limits" partnership, signed days before Moscow sent troops into Ukraine.

Alongside China, Brazil is a member of the BRICS group, a political and economic bloc that Moscow and Beijing see as a counterweight to the West.

The leaders of Vietnam, Mongolia, Egypt and Myanmar -- all long-standing partners of Russia -- are also all in Moscow for the parade, according to the Kremlin.

From Africa, the heads of Burkina Faso, Zimbabwe, the Republic of the Congo, Ethiopia and Equatorial Guinea will be attending.

As will two of Russia's most long-standing partners in South America -- Venezuela's Nicolas Maduro and Cuba's Miguel Diaz-Canel. They both held talks with Putin on Wednesday in the Kremlin.

Indian Prime Minister Narendra Modi was initially expected but will not travel due to the ongoing escalation with Pakistan, according to media reports.

Laos's Thongloun Sisoulith fell ill with coronavirus a few days before and also had to cancel his planned trip, the Kremlin said.