Poland has protested Wednesday's beating of a Polish diplomat in Moscow - the second such attack in four days.
Officials of the Polish Embassy in Moscow delivered a letter of protest to Russia's Foreign Ministry on Thursday.
The ministry has expressed regret over the attack, in which Polish diplomats say attackers beat embassy Second Secretary Marek Reszuta, near the embassy building. Mr. Reszuta sustained non-life-threatening injuries.
On Sunday, thugs attacked another Polish embassy employee, who remains in serious condition at a Moscow hospital.
On July 31 three sons of Russian diplomats were attacked in Warsaw. Poland expressed regret, but rejected Moscow's call for an official apology.
Russian-Polish relations have been strained in recent years over issues from Poland's joining NATO to calls for political change in parts of the former Soviet Union.
Some information for this report provided by AFP and AP.