Police Scotland said inquiries were ongoing into a banner unfurled by Rangers fans and a bottle thrown onto the pitch during the Old Firm match on Sunday.
The display in the home end showed former player Graeme Souness holding a shotgun above a banner reading “Take aim against the rebel scum”.
The picture of Souness is from a Getty Images photoshoot before the World Cup match between Scotland and New Zealand in 1982.
The display was opposite the section holding the 2,100 travelling Celtic fans in what is only the second Old Firm match in recent years to have an away support.
Rangers felt the display, which was organised by fans, was “unacceptable”, “crossed a line” and lacked good judgment.

In a separate incident, what appeared to be an empty Buckfast bottle was thrown onto the pitch, close to the goal being defended by Celtic keeper Viljami Sinisalo.
Sinisalo was seen removing other items from the turf before Rangers substitute Neraysho Kasanwirjo helped by coming on to the pitch and removing the bottle.
Rangers say an investigation is already under way to identify who threw the bottle and that the club will take “the strongest action” against the individual.
A Rangers spokesperson said: “Rangers Football Club condemns the actions of the individual who threw a bottle during today’s match. This behaviour has no place at our stadium. Steps have already been taken to identify the person responsible, and the club will take the strongest possible action once their identity is confirmed.
“We also acknowledge that the tifo displayed prior to kick-off crossed a line and was unacceptable. Our approach to supporter displays has always been built on trust, with the responsibility placed on groups to exercise good judgement.
“We will reflect on how these displays are managed and engage with supporter groups to ensure the right balance of trust, responsibility and oversight is maintained moving forward.
“Rangers is proud of its passionate fanbase, and we all share a duty to protect the standards and reputation of our club.”
On the pitch, the teams shared a 1-1 draw after goals from Cyriel Dessers and Adam Idah.
Police Scotland said enquiries into both incidents are ongoing.
Swinney: ‘Regrettable actions of a minority must be addressed’
First Minister John Swinney says there were “regrettable actions by a minority” at games across the weekend in Scotland.
It comes after a video emerged of a disabled St Mirren supporter being the subject of an alleged hate crime from Aberdeen fans in Paisley.
Police launched an investigation into the incident with both clubs condemning the chanting.
When asked by STV News about the banner at Ibrox, the SNP Leader said: “There’s been some behaviour at football matches over the weekend by a minority – it’s a real shame.
“The overwhelming majority of football fans essentially have to deal with isolated pieces of behaviour which are unacceptable or regrettable and they have to be addressed.”
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