Poland's government says it will protest to Russia after the Polish flag was removed from a World War II memorial in Katyn.
Russian authorities took down the flag from the monument in Smolensk Oblast, which commemorates the 25,000 Polish military officers killed by Soviet forces in 1940.
Its disappearance was noted on social media on Friday by visitors and historians.
The mayor of Smolensk, Andrei Borisov, has confirmed that only the Russian flag is now flying at the entrance to the Katyn memorial.
"There can be no Polish flag on Russian monuments, and even less so after the frankly anti-Russian statements by Polish politicians," Borisov said on VKontakte on Friday.
"I think the Ministry of Culture of the Russian Federation made the right decision in removing the Polish flag. Katyn is a Russian memorial," he added.
The decision to remove the Polish flag comes at a time of heightened tensions between Moscow and Warsaw since Russia invaded Ukraine.
Poland, a member of NATO, has supported Kyiv by supplying it with arms.
The Katyn memorial in western Russia has long been a source of debate, with Moscow having denied any role in the massacre for years.
The bodies of more than 4,000 Polish prisoners of war are buried in the cemetery, which is run by the Russian authorities.
In 2010, the Polish president's plane was on its way to Smolensk when it crashed in a forest, killing all 96 people on board.
Polish officials have reiterated claims that Russia was behind the crash, which killed former President Lech Kaczynski.
Two separate reports by Polish and Russian experts have concluded that the aircraft crashed due to human error amid dense fog.