Police Scotland are investigating sectarian abuse aimed at Celtic players as they arrived at Ibrox for their clash with Rangers on Saturday.
Videos on social media appear to show a number of people, who had gathered outside Ibrox, shouting abuse at the visitors as they came off their coach and entered the stadium.
Celtic this evening described the incident as "unacceptable" and confirmed they had reported it to the police.
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A spokesperson for the club said: "The sectarian abuse suffered by our players and staff is completely unacceptable.
"We have raised this issue with Police Scotland and they have confirmed they are investigating the matter. Clearly, we would hope that all efforts are made to identify those responsible and for all appropriate action to be taken."
Superintendent Stevie Dolan of Police Scotland said: "We can confirm that an investigation is ongoing into abusive comments and singing around the time of the Celtic FC team coach arriving at Ibrox Stadium prior to the football match on Saturday, 2 January.
"This is an active enquiry and no arrests have been made at this stage."
Rangers beat Celtic 1-0 thanks to a Callum McGregor own-goal which sent them 19 points clear at the top of the Scottish Premiership, albeit with three games in hand over their Glasgow rivals.
Scottish Government: Celtic have abused travel exemptions for elite sports
Meanwhile, the Scottish Government have asked the SFA to investigate Celtic for a squad training camp trip to Dubai which is getting underway this week.
Celtic have hit back at the Government, maintaining that the relevant authorities approved the trip when it was arranged in November. The club is permitted to travel to the UAE as part of the elite sports exemption allowed by the Scottish Government.
A Scottish Government statement read: "We would expect the SFA to look into Celtic's trip further - while there are travel exemptions for elite sports which are designed to facilitate international & European competition, if we feel they're being abused, we won't hesitate to remove this privilege."
Deputy First Minister John Swinney told BBC Scotland: "I don't think it's a good idea. I don't think it's a particularly great example to set.
"When we are asking members of the public to take on very, very significant restrictions on the way in which they live their lives, I think we have all got to demonstrate leadership on this particular question.
"They will be allowed back in (to Scotland) but they will have to follow all rules in the process and you know, frankly I don't think it's a very good idea to be doing that at this stage."
Celtic have responded by tweeting: "The training camp was arranged a number of months ago & approved by all relevant footballing authorities & @scotgov through the Joint Response Group on 12th November 2020.
"The team travelled prior to any new lockdown being in place, to a location exempt from travel restrictions. The camp, the same one as we have undertaken for a number of years has been fully risk assessed.
"If the club had not received Scottish Government approval then we would not have travelled."