Russia has demanded an apology, compensation and extra protection from Poland after its embassy in Warsaw came under attack from a nationalist mob.
The rioters were attached to a nationalist march to coincide with the country's Independence Day on Monday. They threw firecrackers, bottles and stones at the embassy, on the southern outskirts of the city centre, and torched a police guards' cabin. Police said 72 people were arrested, and 14 went to hospital with injuries.
The two countries share a tragic and bloody history which stretches back hundreds of years. Particular bitterness still surrounds Soviet domination of communist Poland from 1945 until the 1980s, and the Katyn massacre of 1940, in which thousands of Polish prisoners of war were slaughtered.
The Polish ambassador in Moscow was summoned to the Russian Foreign Ministry on Tuesday, where the Russians insisted Poland take steps to punish those responsible and "prevent a repeat of such provocations in the future".
Poland's Foreign Ministry expressed deep regret about the incident and said such behaviour deserved "strong condemnation".
Police used rubber bullets to break up the mob of far-right youths, but Russia said "passivity and belated action by the police" were largely to blame.
The day had started peacefully with a "Together for Independence" march, attended by dignitaries including President Bronisław Komorowski, and a ceremony at the tomb of the Unknown Soldier.
But, according to local press, shortly afterwards came the nationalist "March for Independence".
According to Polish news websites, police spokesman Mariusz Sokołowski said: "We are dealing with a group of several hundred people who are intent on breaking the law."
And Prime Minister Donald Tusk said: “This situation is unacceptable – this kind of event cannot take place. Those who tolerate and accept such events will bear responsibility.” He also suggested such marches might be banned in future.
But Artur Zawisza, one of the leaders of the march, which is spearheaded by the All-Polish Youth and the National Radical Camp (ONR), said: “No piece of paper from the Town Hall will stop us marching towards independence.”
The nasty scenes occurred as international delegates gathered in the city for the UN's climate conference. The high-profile gathering is at the country's national stadium.
The main target of the rioting appeared to have been any symbol of left-wing, liberal views, with the rioters setting fire to a rainbow sculpture - a clear symbol of tolerance towards homosexuals in the strongly Catholic country.
According to Reuters news agency, some Russian officials saw the violence in the context of strains between Russia and the EU over human rights and democracy as Ukraine prepares to sign a trade pact with Brussels that would mark a symbolic move away from Moscow's orbit.
One suggested the unrest showed the problem of nationalist violence is more serious in the EU than in Russia, where anti-migrant rioting rocked Moscow last month.
"The events in Warsaw show: Nationalism is immeasurably stronger in several EU countries than it is in Russia," Alexei Pushkov, the head of the international affairs committee in the lower house of parliament, said on Twitter. "The EU should not lecture us but deal with its own members."
Mikhail Margelov, the head of the equivalent committee in the upper chamber, said the violence had "turned Poland from an influential member of the European Union into ... a third world country," Interfax news agency reported.
Poland has been a strong supporter of closer EU integration with neighbouring Ukraine before a summit in Lithuania on Nov 28-29, at which Kiev could sign an association agreement and develop trade ties with Brussels.