Hackers posted anti-Vladimir Putin messages on Russian television on Monday, telling the Russian president "you have blood on your hands" in response to the Ukraine crisis.
Russian satellite television menus were altered to show viewers in Moscow messages about the war in Ukraine.
Screenshots show Moscow satellite TV menus on Victory Day, when Russia celebrated the 77th anniversary of the Soviet Union's victory over Nazi Germany, with anti-war slogans written across every channel.
"You have the blood of thousands of Ukrainians and hundreds of dead children on your hands," said the slogan.
"The TV and the authorities are lying. No to war."
The messages appeared just before the Victory Day parade on Red Square at which Putin compared the war in Ukraine to the Soviet battle to defeat Adolf Hitler in the Second World War.
In his speech, Putin said Russian forces in Ukraine were fighting for the future of their motherland. The annual memorial day celebrates Russia's victory over Nazi Germany.
Despite rumours he would make a major announcement in his speech, the Russian leader stuck largely to defending Russia's invasion.
He told his audience of millions: "Today you are fighting for our people in Donbas, for the security of Russia, our homeland."
Defending the Russian presence in Ukraine, he said that Nato and Ukraine had been "preparing a punishing operation in Donbas to intrude on our historic lands".
He said: "In Kyiv they were saying they might get nuclear weapons and Nato started exploring the lands close to us, and that became an obvious threat to us and our borders."
Putin linked the war in Ukraine to victory in 1945.
Experts who watched Putin's Victory Day speech described it as a "professional performance" that showed no "potential craziness" that Western officials have recently been concerned about.
Former UK ambassador to Russia Sir Tony Brenton told Sky News Putin had recently shown "disturbing signs" of "losing it", in his speeches surrounding the war, but the speech on Monday was "coherent" and a "very professional performance".
He added the speech had "pushed all the right patriotic buttons in the Russian political atmosphere".
It was not immediately clear how the hackers' slogans appeared on Russian screens. Interfax news agency said they also appeared on cable television.
A Russian news website also showed anti-war material that was deeply critical of Putin. It has not yet been established as to how the negative articles appeared, but they were swiftly removed.
Monday was not the first time hackers have managed to infiltrate Russian media.
In February, the Anonymous hacking group declared a 'cyber war' on Russia following its invasion of Ukraine.
Posting on its Twitter account, which has more than eight million followers, the group claimed responsibility for disabling prominent Russian government, news and corporate websites and leaking data from entities including the federal agency responsible for censoring Russian media.