The Russian foreign ministry has demanded an apology from Poland after nationalist rioters attacked the Russian embassy in Warsaw on Monday.
It conveyed the demand after summoning the Polish ambassador to Moscow, Wojciech Zajaczkowski.
Poland's foreign ministry has expressed "deep regret" at the incident, blaming it on ultra-nationalists.
Rioters threw fireworks at the embassy and burnt a rainbow-coloured arch symbolising tolerance.
Polish police used rubber bullets to break up groups of masked youths from an ultra-nationalist march which attracted thousands of supporters.