Sergey Andreev had planned to pay tribute to Soviet soldiers at a Warsaw cemetery as Russia marks Victory Day.
Russia’s ambassador to Poland was hit with red paint thrown as he prepared to mark a World War II anniversary in Warsaw.
Sergey Andreev was met by hundreds of protesters as arrived at the Soviet Soldiers Cemetery in the Polish capital.
As Russia celebrates Victory Day, Andreev had planned to pay respects at a Warsaw cemetery to Red Army soldiers who died during the war.
But his entourage was blocked by activists, who then doused the ambassador with red paint.
The protesters carried Ukrainian flags and chanted “fascist” and “murderer” following Russia's invasion of Ukraine on 24 February.
Other activists were dressed in white sheets smeared with blood, symbolising the Ukrainian victims of Russia’s war.
Police arrived at the scene to help the ambassador and other members of his delegation leave.
Andreev told reporters that he and his team were not seriously hurt in the incident, according to the TASS news agency.
On Sunday, anti-war protesters had also demonstrated outside the Russian embassy in Warsaw, parking a tractor in front of the complex.
Poland has long advocated for stronger European Union sanctions on Russia over its invasion of Ukraine and authorities in Warsaw had initially warned the Russian Embassy about a planned Victory Day march.
"We will make a formal protest," Andreev told reporters on Monday.
"When they recommended that we not hold a larger event, we met them halfway, we didn't aggravate the situation."