The 126-year-old rivalry between supporters of Glasgow's two biggest clubs, Celtic and Rangers, has often spilled into sectarian bigotry away from the football pitch.
Last November Neil Lennon had to appeal to Celtic fans to leave political statements outside the club's Parkhead stadium.
It came after some fans had unfurled banners featuring Scottish historical figure William Wallace and IRA hunger striker Bobby Sands during a Champions League fixture with AC Milan.
Lennon, who was the victim of sectarian abuse from a section of Northern Ireland fans – and who received a loyalist death threat after he joined Celtic as a player – said: "I don't know what you can do about it. It is certainly not welcome within the stadium."
Last September a Celtic fan was convicted of engaging in "sectarian and offensive conduct" after singing The Roll Of Honour during Celtic's clash with Dundee United.
The man was banned from every football ground in the UK for three years and fined £600 after being caught on camera singing 20 to 30 seconds of the song with other supporters.
In October 2011 Celtic urged fans to stop singing pro-IRA chants after being "inundated" with supporters' complaints.
Police announced they had launched an investigation into the singing of sectarian songs following a game against Hearts.
Celtic chief executive Peter Lawwell said at the time: "Chants glorifying the Provisional IRA are totally unacceptable."
During his time in Glasgow, Lennon has suffered attacks in his car, death threats daubed on the road outside his home, he was knocked out in a street attack, had bullets sent to him in the post, was attacked by a Hearts fan at a game, and was a victim of a letter-bomb campaign that saw two men jailed for five years.