UEFA says Porto have been charged with racist behaviour by fans at a Europa League game.
Abusive shouts were allegedly targeted at Young Boys player Jean-Pierre Nsame, who is black, as he prepared to take a penalty kick in the 15th minute of Thursday's game.
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Nsame, a Cameroon international, scored from the spot in the Swiss team's 2-1 loss in Portugal.
The incident was highlighted Friday by a television commentator for the French-language Swiss public broadcaster.
UEFA says its disciplinary panel will judge the case next Thursday.
The case follows a similar incident in Italy this month when Cagliari fans aimed racial insults at Inter Milan forward Romelu Lukaku as he took a penalty.