The video, which is being widely shared online, appears to show a police officer hitting a young Barnsley fan over the head with his baton.
The person who shared the video, Spencer White, said two fans had been fighting, but that the reaction of the officer was totally over the top.
The young fan who is hit then falls to the floor before blood begins pouring from what appears to be a serious wound to his head.
Writing on Facebook, Spencer wrote: “Share everywhere I thought they were supposed to protect people. Disgusting.”
“Two lads were scrapping and then he runs in and hits one in the head which was uncalled for completely.”
In the match itself – which kicked off at 1.30 on police advice – the fierce local rivals drew 1-1.
On Twitter, South Yorkshire Police said: “Following the Barnsley V Sheffield Wednesday match today (Saturday 8), our officers responded to disorder between fans outside of the Interchange.
“An investigation is underway into the circumstances surrounding the incident.”
The video has been shared thousands of times already and attracted hundreds of reactions.
On the Sheffield Star Facebook page most readers condemned the officer's actions unequivocally.
Darren Lindley said: “No other profession gets away with this. Absolutely shocking.”
Dean C. Long said: “The copper should be charged and sacked!”
And Lynn Buckley said: "Get this viral and justice for this lad."