Poland on Saturday said it had seized a high school building near Moscow's embassy in Warsaw meant for the children of diplomats, a move the Russian envoy called "illegal".
The spat over the 1970s multi-storey building has been going on for a year.
"This building belongs to the Warsaw City Hall," Polish foreign ministry spokesman Lukasz Jasina told AFP, adding that the move followed a bailiff's order.
The spokesman for the municipality was unavailable for comment.
Poland says there is a huge disparity in the number of diplomatic buildings each had in the other country.
"We regard this as yet another hostile act by the Polish authorities and a blatant violation of the Vienna Convention of 1961," Russia's foreign ministry said.
The ministry promised a "harsh reaction and consequences for Polish authorities and Poland's interests in Russia".
Moscow's envoy, Sergei Andreyev, also denounced Poland's intervention as an "illegal" intrusion on a diplomatic facility, saying teachers and staff lived on the building's grounds.
The school would continue to work in a different part of the Russian embassy's premises, he added.
"Our priority is ensuring the safety and the interests of our employees and their families."
Poland's foreign ministry issued a statement in March 2022, shortly after Russia launched its invasion of Ukraine, saying the Polish courts had ruled that Russia had to return the buildings to Polish control.