CSKA Moscow soccer fans chanted racist abuse at Arsenal's black players several times during a Europa League match late on Thursday, a Reuters witness said.
A Reuters photographer at pitch level heard dozens of home supporters make monkey chants at Arsenal's Danny Welbeck and Alex Iwobi when they neared a particular section of the stands when the ball went out of play.
The chants were also heard when Welbeck celebrated his 75th minute goal that brought Arsenal back into the game from 2-0 down, the photographer said. The game ended 2-2.
A third Arsenal player who did not come off the substitute's bench was targeted with the same chants after the final whistle as he jogged to warm down on the pitch at the CSKA Arena, the witness said. The player, who Reuters was unable to identify, was black.
Russia faces increased scrutiny over fan behaviour ahead of the World Cup it hosts starting in June and the country has pledged to crack down on racism.
UEFA told Reuters on Friday that no racist behaviour had been reported to it by match officials at the game.
UEFA said it had opened disciplinary proceedings against CSKA Moscow on Friday for infractions including throwing objects and setting off fireworks. The alleged infractions were not connected to racist or discriminatory behaviour.
The European soccer body said it took allegations of racist behaviour very seriously and it would be grateful for audio or video evidence of any other incident as it required sufficient proof to be able to press charges.
The Russian Football Union declined to comment. Reuters made repeated calls to a mobile number for a CSKA spokesman but there was no reply. An email to the club was not answered.
A spokesman for Arsenal declined to provide a comment for publication.
Football governing bodies have previously taken action in response to racist and discriminatory behaviour at Russian soccer grounds on several occasions.
UEFA last month opened disciplinary proceedings against Saint Petersburg's Zenit over racist chanting and other violations by fans at a Europa League game against Leipzig.
FIFA said last month it was probing racist abuse directed at French players by Russian fans during a pre-World Cup friendly between the countries in Saint Petersburg.