Vladimir Putin has used his annual Victory Day speech in Moscow's Red Square to justify his war in Ukraine and denounce Nato.

Speaking in front of hundreds of military personnel, the Russian president said he was fighting a "just" war and called Ukraine an "aggressive force" that is being "armed and supported by the whole bloc of Nato".

His remarks come amid muted celebrations across Russia to mark the nation's biggest national holiday, commemorating the USSR's victory over Nazi Germany.

Ahead of the celebrations, curtailed for security reasons, Russia and Ukraine agreed to observe a three-day ceasefire, announced by US President Donald Trump on Friday.

Following the parade, Russia's defence ministry accused Ukraine of breaking the ceasefire, without providing details. Ukraine did not immediately comment.

For the first time in years, no armoured vehicles or ballistic missiles featured in Moscow's Victory Day parade.

But under tightened security, military personnel marched in droves across Red Square.

Addressing the crowd, Putin started his speech by commemorating the sacrifices of Soviet soldiers during World War Two. 

"The great feat of the generation of victors inspires the soldiers carrying out the goals of the special military operation today," he said, referring to the war in Ukraine which has been going on for more than four years.

"They are confronting an aggressive force armed and supported by the whole bloc of Nato. And despite this, our heroes move forward."

The Russian leader went on to celebrate the citizens of Russia, making reference to the contributions of workers to its war effort, including scientists, inventors, military correspondents, doctors and teachers.

"No matter how military tactics change, the future of the country is being provided for by the people," he said.

Immediately after the speech, cannons fired successive rounds before music was played by a military brass band.

Foreign guests in attendance included Belarus's leader Alexander Lukashenko, Malaysia's King Sultan Ibrahim and Uzbekistan's President Shavkat Mirziyoyev.

Slovak Prime Minister Robert Fico, the only representative of the EU to attend, was also pictured meeting Putin at the Kremlin ahead of the parade. 

Markedly fewer world leaders turned up compared to last year's 80th anniversary parade, which featured 27 leaders including China's President Xi Jinping and Brazil's Luiz Inácio Lula da Silva.

Russian TV viewers were shown a broadcast of soldiers on the front lines after Putin's speech.

Following the ceremony in Red Square, Putin laid flowers at the Tomb of the Unknown Soldier before attending a reception at the Kremlin.

Victory Day celebrations took place in other parts of Russia, though in fewer numbers than previous years.

In far-eastern Vladivostok, a parade and an Immortal Regiment march honoured war veterans.

Marches also took place in a number of other cities including Krasnoyarsk, in Siberia, and St Petersburg, state media reported.

Other parades were called off altogether. 

Some virtual events were scheduled to take place instead, though internet outages were expected to disrupt them.

Under Putin, Victory Day has been used a propaganda tool to show off Russia's military might.