There was no initial police presence at the trouble-hit Manchester Under-21 derby, it has been confirmed.
Disorder in the north-east corner of Old Trafford between United and City fans was first reported by a spectator inside the stadium at about 20:50 BST.
Greater Manchester Police said it was examining CCTV footage of the incident.
A 15-year-old boy and a man, 19, were arrested outside the stadium on suspicion of assault. Both have been released pending further inquiries.
A 13-year old boy was arrested after going on to the pitch, police said, and he will be dealt with by summons.
A police spokesman told BBC News their presence had not been requested before the game by Manchester United.
Manchester City fan Andy Savage was at the game - attended by about 16,000 fans who were attracted by a free admission offer - with his son.
He told BBC Radio Manchester: "After half an hour, United fans were streaming in and the stewards were putting them closer and closer to our left, where we were being boxed in.
"Fights broke out and basically it was a free-for-all. There were kids screaming in the City end."
He said fans "were being kicked and trampled on and it was very, very ugly scenes".
Manchester United has not commented but Manchester City said it was "aware of crowd disturbances at Old Trafford last night involving both sets of supporters".
A spokesman said: "Club officials are liaising with Greater Manchester Police and Manchester United over this situation and will assist with their enquiries."
United won the Under-21s Premier League game 4-0. They are currently top of the table and need one point from their final game - a return fixture against City - to secure the title.
http://www.bbc.co.uk/news/uk-england-manchester-32716950
