The child was caught in the crossfire of a fight during an Orange Order march, leaving her needing stitches to her face.
Officers want anyone who photographed the incident on their mobile phones or cameras to send the footage to a dedicated email address.
Police have set up the account GlasgowGreenAppeal@scotland.pnn.police.uk specifically for this purpose.
Detective Sergeant Ronnie Payne at London Road CID is appealing for witnesses.
He said: "This was a particularly nasty incident which left a 12 year old girl with a head injury that may leave scarring. I know that Glasgow Green was very busy on Saturday afternoon with bands, followers and members of the public.
"Many people would have been taking pictures on their mobile phones or cameras and may have images that would assist police.
"I would urge people who were in the park and took photos to check if there are any that caught the disturbance or the girl being struck.
"If so, I would urge them to email them to the dedicated email address."
The girl was assaulted with what is believed to be a glass bottle on Saturday, July 5.
She has been in the park with a 22-year-old female relative to watch the Orange Walk when a fight broke out.
The girl was taken to Yorkhill Hospital where she received stitches to a head injury then later released.
Mr Payne added: "I would also ask anyone who has information and has not yet come forward, to email police at Alternatively telephone the CID at London Road on the non emergency number 101 or Crimestoppers on 0800 555 111 where anonymity can be maintained."