The Champions League game between Paris Saint-Germain and Basaksehir was interrupted when players walked off the field on Tuesday after alleging fourth official Sebastian Coltescu of Romania used a racial slur against an assistant coach.

Players from the Turkish team were furious after assistant coach Pierre Webo was shown a red card by referee Ovidiu Hategan of Romania at Parc des Princes, saying Coltescu had used a racial term against Webo, who is from Cameroon, before he was sent off.

Basaksehir substitute Demba Ba demanded that the fourth official explain himself, while PSG players Neymar and Kylian Mbappe also demanded an explanation. Basaksehir coach Okan Buruk said “you are racist” to Coltescu.

The scored was 0-0 when the incident took place about 15 minutes into the match.

Webo was enraged and was heard to repeat at least six times “Why you say negro?” as he sought an explanation from Coltescu.

Moments later, Ba came off the bench and stood in front of Coltescu and said: “Why when you mention a black guy, you have to say ‘This black guy?’”

UEFA replaced Coltescu and pledged to investigate.

“Following an alleged incident involving the 4th official, the match was temporarily suspended. After consultation with both teams, it was agreed that the match would restart with a different 4th official,” UEFA said in a statement.

“UEFA will thoroughly investigate the matter and further communication will be made in due course.”

During the interruption, Basaksehir posted a message on Twitter against a UEFA backdrop with the message “NO TO RACISM.”

Seven years ago, Hategan was in charge of a match between CSKA Moscow and Manchester City when City’s black players were racially abused.

City captain Yaya Toure directed referee Hategan toward fans making monkey noises at the English club’s black players.

Michel Platini, UEFA’s president at the time, requested an internal inquiry involving the referee to examine why guidelines were not followed to respond to discrimination incidents during matches.