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".
This prompted Russia to promise it would respond harshly.
The spat over the 1970s multi-storey building, nicknamed the "spy nest" by Warsaw citizens, 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.
Jasina said that it was Russia's right to protest but that Poland was acting within the law.
"Our opinion, which has been confirmed by the courts, is that this property belongs to the Polish state and was taken by Russia illegally," he said.
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.
Russia's Foreign Ministry said in a statement that the Polish authorities had burst onto the embassy school's grounds with the aim of seizing it.
"We regard this latest hostile act by the Polish authorities as a blatant violation of the 1961 Vienna Convention on Diplomatic Relations and as an encroachment on Russian diplomatic property in Poland," the ministry said.
"Such an insolent step by Warsaw, which goes beyond the framework of civilised inter-state relations, will not remain without a harsh reaction and consequences for the Polish authorities and Polish interests in Russia," it said.
Earlier in the day, Moscow's envoy, Sergei Andreyev, termed the development an illegal act. “An intrusion on a diplomatic facility."
Andreyev said this was a "violation of Vienna's convention of diplomatic relations."
"Of course, there will be a reaction," Andreyev said, adding that he did not want to give further details and that a decision will be made in Moscow.
"We consider the school by the embassy to be part of our diplomatic mission," he said, saying teachers and staff lived on the territory of the building.
Andreyev said the school will continue to work in a different part of the Russian embassy's premises. "Our priority is ensuring the safety and the interests of our employees and their families."
The two countries' already fraught relations have soured further over the war in Ukraine with Warsaw positioning itself as one of Kyiv's staunchest allies, playing a leading role in persuading allies to provide it with heavy weaponry.
