One Celtic fan was taken to hospital and four others were injured in an overcrowding incident at Celtic Park described as "frightening" before Sunday's Old Firm win over Rangers.
Hundreds of fans were stuck outside the North Stand and missed the opening exchanges of Celtic's 1-0 victory over their Glasgow rivals.
The incident concerned Celtic fans accessing the stadium and police confirmed they will work with the club to prevent it from happening again, but some fans felt more could have been done by the authorities present.
Celtic fan Martin Lennon told Sky Sports News: "The scenes were shambolic. After being refused access down London Road, I tried to go through the Janefield Street tunnel - the main route round the stadium.
"When I got there I found myself among thousands of supporters who were blocked from using the tunnel. There was no warning, no explanation, nothing over the PA system, no details about alternative routes and only a handful of police.
"All they did was close the tunnel and left everyone to work it out for themselves. Tens of thousands of people use that tunnel on match days. It is the main route around the ground.
"I saw hundreds of people climbing into the Eastern Necropolis to get around the stadium and no police or security giving any alternative."
Another Celtic fan, Mike Robb, described the "frightening" scene as people climbed walls and fences to avoid the crowds.
He said: "It was unpleasant then pretty frightening. Kids were being put up on shoulders and a lot of people were starting to climb the wall and fence into the old graveyard to get out of the crush.
"There was no stewarding to control access or advise on alternative access routes. Police were just sitting on their horses at either end of the passage.
"When I got out I tried to tell them there was a problem but just got told to move on. I'm standing there in the middle of the passageway feeling more and more people crushing in around me."
In a statement released by Celtic on Sunday evening, the club expressed gratitude for their supporters' patience.
"We are aware of the issues caused by congestion prior to kick-off," the statement read.
"We are pleased that stewards and police were able to assist quickly and we thank our supporters for their patience shown while the matter was being dealt with.
"We will work closely with police to investigate and review this matter. The safety of our supporters will always be our priority."
Chief Superintendent Brian McInulty of Police Scotland confirmed: "Five people were treated by first aiders when fans were attempting to get into the stadium in the lead up to kick-off at Janefield Street.
"Four people were treated at the scene and then went into the ground, and the other was taken to hospital after a fall from the wall bordering Janefield Street. This was a dynamic situation, occurring 10 minutes before the match started.
"Officers and stewards reacted quickly upon realising there was an issue and put in place measures to relieve the congestion. This included putting in place cordons to prevent further entry at Janefield Street and opening up London Road to allow fans to access the stadium from the south.
"We work closely with Celtic Football Club to ensure the safety of all fans attending matches. We plan and practice various scenarios to ensure that if an incident occurs it will be dealt with as quickly as possible, as happened in this case. We will review today's incident and work with Celtic to ensure any learning is quickly put in place."
Celtic confirmed via their Supporter Liaison Officer, John Taylor, that they are looking into the incident.
He wrote on Twitter: "Apologies to fans affected by overcrowding issue at North Stand earlier, the club will be investigating the cause to ensure no recurrence."