An investigation is under way after four football fans were injured in a crush ahead of an Old Firm match at Celtic Park.

The incident occurred on Janefield Street, outside Parkhead, shortly before the game began on Sunday afternoon.

Fans were treated by paramedics before heading inside to watch Celtic's clash with Rangers.

Meanwhile, one man was taken to hospital after falling from a wall at the stadium.

A police investigation is under way into the cause of the crush, it is understood.

Celtic, meanwhile, apologised to fans caught up in the incident.

It was the first Old Firm match at Parkhead since Celtic cut ticket allocation for Rangers fans from 7000 to 800. It followed a similar move by Rangers.

Announcing the decision in July, a Celtic spokesman said the club would "closely monitor the implications for crowd management".

On Sunday, Police Scotland chief superintendent Brian McInulty said: "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 to ensure the safety of all fans attending matches."

He later added: "Following the events that took place before yesterday's Celtic v Rangers SPFL match, tomorrow I will be meeting with representatives of Celtic FC and the other emergency services to undertake a thorough and comprehensive de-brief of the policing and stewarding operation.

"Later this week we will also be happy to meet with representatives of Celtic FC supporters groups to discuss any concerns they may have.

"The safety of all those attending matches remains the absolute priority for Police Scotland. The situation yesterday was a fluid one and decisions were taken quickly to prevent any further persons from gaining access to Janefield Street and to alleviate the congestion in this area.

"The response to the overcrowding was in line with tried and tested plans involving stewards, police officers on foot and mounted officers. There has been speculation in the media regarding a gate having been closed on Janefield Street and that this contributed to the congestion.

"At this time we believe that the gate was open at all times in the lead up to the match, however as this has been raised as a concern I will ensure that this forms part of the review."

Celtic also issued a statement following the incident on Sunday, in which they apologised to fans.

It said: "The safety of our fans will always be of paramount importance and this is a matter which we are treating with the utmost seriousness.

"The club, along with Police Scotland, will be investigating this matter fully to identify the causes and take the appropriate steps to ensure this does not happen again at Celtic Park.

"The club and Police Scotland will advise of the outcome of this investigation once it is complete.

"We are fully aware of the serious difficulties which arose yesterday for supporters, and we sincerely regret and apologise that so many fans were faced with this situation.

"Yesterday's match was the first occasion which featured new segregation and access arrangements.

"Our total focus now is to work with the police in looking at these arrangements and other circumstances surrounding this particular fixture and to take the correct action to ensure this situation is never repeated.

"Our priority will always be to ensure our supporters enjoy a safe and positive experience at Celtic Park. We will be doing all we can to make sure this continues to be the case at all times."