This is the moment fans began fighting at an under-21 derby between Manchester City and Manchester United at Old Trafford.

United’s 4-0 victory was tarnished after stewards were forced to intervene when brawling supporters began throwing punches.

Police have said three teenagers were arrested as the trouble broke out.

This video, posted on the @Footy_Fooligans Twitter account, shows a number of fans jumping over barriers to exchange blows.

Security staff were forced to intervene to keep fans apart during the match, with one supporter invading the pitch.

Trouble broke out in the J stand at Old Trafford as stewards confronted jeering fans in an attempt to escort them back to their seats.

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READ: Match report

Two groups shouted across the stand amid heated scenes, while concerned fans in other parts of the stadium watched on.

United eventually won the game 4-0 in front of a 16,000-strong crowd.

A man also ran onto the pitch while the game was being played.

Separate footage taken by fans shows the lone intruder racing across part of the turf.

He was later shown off the pitch by United player Liam Grimshaw.

Greater Manchester Police said officers detained a 13-year-old boy inside the ground on suspicion of pitch encroachment.

It was later decided he would be dealt with by summons.

A 15-year-old boy and 19-year-old man were also detained outside the ground on suspicion of assault but both have now been released pending further enquiries.

The M.E.N understands that under-21 matches are not routinely policed like normal matches and there was no request from United for this match so there was no operation in place.

But one fan said the fact it was free admission should have warranted a better security operation.

Match highlights in pictures

He also claimed he heard chants mocking the Munich air disaster during the disturbances.

The fan, who did not want to be named, said: “Action needs to be taken.

"There were 16,000 people there and it was fantastic to see so many families and children.

“But it was disgusting to see some fans behaving this way.

"I heard chants about the Munich disaster which is so distasteful and there was also taunts from the United fans. There was no police to be seen when the trouble started.”

The two teams are set to play each other again at City’s Etihad campus within the next few weeks.

A United spokesperson issued a statement to the M.E.N: "There was a minor incident at last night's U21 game across segregation lines from a small number of fans after United scored the 3rd goal.

"Stewards resolved the matter quickly. The fans that have been identified will be dealt through the usual channels.

"The Club worked with Manchester City in advance to make sure segregation was effective.

"Police reported some minor disturbances outside the ground after the match."