South Korean international footballer Eom Ji-sung was allegedly racially abused during Swansea City's 1-0 win at Millwall in England's second-tier Championship.
The 24-year-old winger was warming up in the second half as a substitute for visitors Swansea on Tuesday evening when the alleged incident happened.
The game was stopped for about seven minutes as the referee spoke to both coaches, followed by an announcement over the loud speakers at The Den stadium in southeast London, reports said.
The announcement said there were alleged chants of racism and that police were scouring CCTV to identify who was responsible, the BBC said.
Swansea said Eom, who recently won Asian Games gold with South Korea and played for his country at the World Cup, was the victim.
"As a club we condemn racism and discrimination in all forms, and there is no place for such behaviour in football or in society," the Wales-based team said in a statement.
"The club wants to place on record its thanks for the swift and decisive manner in which the match officials, Millwall Football Club and the police dealt with the incident."
Swansea, who top the Championship, added: "We stand with Ji-sung and he has our unstinting love and support."
Millwall said it was "aware of an alleged, individual incident of discriminatory abuse towards a Swansea City player".
"The club operates a zero-tolerance policy towards any form of discriminatory abuse," it said in a statement.
"Any individual found to have engaged in such behaviour will face the strongest possible action."
A police investigation was under way, the club said.