Poland was in hot water on Tuesday after ultra-nationalist rioters went on the rampage outside the Russian embassy in Warsaw, igniting a diplomatic row with Moscow.
The violence, which marred Polish independence day on Monday, reflects growing far-right and anti-Russian sentiment in Poland, an ex-communist country and European Union member.
The havoc erupted as between 20,000 and 50,000 people surged through Warsaw in a march organised by far-right groups, many wearing balaclavas and carrying lighted flares.
The protest turned violent as rioters set alight two cars and a guard's booth outside the Russian embassy. Twelve police officers were hospitalised after demonstrators threw stones and bottles, while 72 protesters were detained.
The Russian foreign ministry on Tuesday summoned Polish Ambassador Wojciech Zajaczkowski and demanded an apology.
It also criticised what it labelled a delayed police response to the unrest and called for the "punishment of the guilty and for such provocations not to be allowed to happen in the future", according to a ministry statement.
Russia's ambassador in Poland, Alexander Alexeyev, for his part requested a meeting with the foreign ministry and told reporters he regretted the "permanent, hysterical anti-Russian campaign" in Poland.
The Polish foreign ministry expressed its "deep regret over the violent behaviour and incidents that occurred near the embassy."
The two countries have a history of complicated relations marked by centuries of conflicts. Post-Communist Poland's integration into the West as an EU and NATO member is an ongoing source of tension.
That antagonism often bubbles to the surface on the anniversary of November 11, 1918, when Poland won back its independence after being wiped off the European map for 123 years in a three-way carve-up between Russia, Prussia and the Austro-Hungarian Empire.
An official independence day event went off without a hitch on Monday, before the far-right groups went on the rampage, prompting Polish foreign ministry spokesman Marcin Wojciechowski to tweet: "There is no justification for hooliganism."
The groups included the marginal All-Polish Youth and the National Radical Camp, whose members are mainly football hooligans.
Many demonstrators demanded the resignation of Prime Minister Donald Tusk's unpopular centre-right government, while others evoked assassination conspiracy theories over a plane crash that killed Polish president Lech Kaczynski in 2010.
Anti-Russian sentiment has increased since the presidential jet went down in thick fog while approaching Smolensk airport in Russia. The crash killed 96 people including Kaczynski, brother of the current head of the conservative Law and Justice opposition party.
The groups also espouse euroscepticism and oppose abortion and gay marriage.