Children as young as 10 and a police officer have been injured during violent clashes between Sheffield United and Middlesbrough fans.
More than 200 supporters of both teams were involved in a mass brawl following their Championship game at the Riverside Stadium yesterday evening.
Cleveland Police said the violence began after United fans invaded the pitch to celebrate a late goal, which was disallowed, and then the clashes continued outside the ground in the car park.
Police and stewards attempted to keep order, however fans as young as 10-years-old were caught up in the incidents.
A police officer also had to be taken to hospital with injuries sustained during the clashes.
Five people have been arrested on suspicion of a number of offences and are in police custody.
But the force warned that the number of arrests is expected to rise as the investigation unfolds.
Jason Harwin, a former South Yorkshire Police officer and current assistant chief constable of Cleveland Police, said: 'We will absolutely not tolerate violence of this nature and we will work to identify the offenders and bring them to justice.
"The vast majority of fans accept that football is a family game, watched by children, parents and grandparents. It's terrifying for children to witness such violence, or be innocently embroiled in it as their parents frantically try to keep them safe.
"Unfortunately a small minority of people behave in a manner which puts innocent fans at risk. We will be working closely with Middlesbrough and Sheffield United as part of our investigations."
Middlesbrough won the game 1-0 after Rudy Gestede's first half header.
Anyone with information that may help police are asked to call Cleveland Police on the non-emergency number 101.
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