Celtic have launched an internal investigation into Leigh Griffiths' behaviour before last weekend's Edinburgh derby after police began making enquiries into allegations of "racist chanting" by the striker.
Celtic last week fined the former Hibernian striker after footage showed him singing "Hearts are going bust" in a pub before attending the Leith side's 2-0 defeat at Tynecastle.
But new footage emerged on Sunday which appears to show the Hoops forward singing and dancing as Hibs fans sing a derogatory song about former Hearts player Rudi Skacel.
A Celtic statement on Monday read: "Due to an ongoing police investigation we are unable to comment. However, the club takes the allegations extremely seriously and have launched our own internal investigation."
Police Scotland confirmed on Sunday evening that it was investigating the incident.
A spokeswoman said: "Following an article in a Sunday newspaper alleging racist chanting by a 23-year-old Scottish Premier League footballer, inquiries are being initiated by Police Scotland."