Police have warned Celtic fans attending a rally against Scotland’s football laws that any attempt at a march would be illegal and see participants prosecuted.

Organisers are predicting several thousand will attend Fans Against Criminalisation (FAC) protest in Glasgow’s George Square at Saturday lunchtime, organised on the back of the incident which saw Celtic’s self-styled ‘ultras’ the Green Brigade clash with police last month.

The group were accused by Strathclyde Police of attempting to organise an unauthorised march against the Offensive Behaviour at Football legislation when disorder erupted on March 16, sparking 13 arrests and a week of fall-out and criticism of police tactics.

Earlier this week, Glasgow City Council declined to grant permission for FAC to parade through the city centre on Saturday, as the organisers had not complied with the required 28 days' notice to hold a street procession.

However, permission for the group to hold a rally in George Square from 12 noon until 1:30 pm was granted. Although formal approval is not normally required for static protests, the fact that George Square is officially deemed a city park means permission has to be granted by the landowners, the council.

But while Police Scotland said it “understands and supports the decision of the council” and is “working with it and others involved in the rally to make sure the day is a success”, it has warned against the potential for an impromptu march to Celtic Park, where the club could become SPL champions if they beat Hibs and Motherwell lose at home to St Mirren.

Chief Superintendent Andy Bates, Divisional Commander, Greater Glasgow Division, said: “Clearly, as permission for a public procession has not been granted, those attending the rally need to understand that any procession along the road from George Square would be illegal and anyone involved could find themselves subject to prosecution.

“The way to avoid this problem and reduce the impact on the city centre and the local community is for people to leave the square using the footpaths on the three identified routes.”

Mr Bates added he was “slightly concerned” about the dispersal at the end of the rally, with large numbers of fans wanting to leave the square at one time and making their way to Celtic Park.

He said: “It is important for their own safety and the safety of other road users that they don’t block the roads as they leave the square, and to reduce the risk of congestion we have identified three separate routes to the stadium leaving from George Street, Cochrane Street and South Frederick Street on to Glassford Street. Using these three routes will allow supporters to leave safely and quickly, using the footpaths and not blocking the streets.”

Police Scotland has also put a traffic management plan in place that will allow supporters using buses to be dropped off in George Square before the rally. Buses will then be able to park in George Street to take supporters to Celtic Park once the rally is over.

Mr Bates also advised those attending “to wrap up warm and come along and enjoy their day”, encouraging anyone seeking advice “to speak to my officers in the square”.

A source at Glasgow City Council said: “The FAC group submitted notification for a public procession, but not within the statutory period. We declined, as there were no exceptional circumstances re the choice of date. If you remember, the argument re the Rangers one to Hampden was that the club may no longer have existed in 28 days.”

FAC said confirmed speakers at the rally are MSP Michael McMahon and Glasgow University academic and Celtic Trust member Jeanette Findlay.