A 15-year-old boy was beaten up by a group of eight to 12 teenagers in St Albans' Verulamium Park.
Police are appealing for witnesses after the attack at around 4.30pm on Monday 14 June.
The victim was in the park with his friends when he was attacked by the group - who punched and kicked him causing cuts and bruises.
The group are described as teenage boys aged 17 to 18 years old.
One is described as white and was riding an orange mountain bike. The rest of the group are described as either mixed race or black.
PC John Cooper, who is investigating, said: “This was a frightening ordeal for the victim and I would like to reassure the public that we are carrying out thorough enquiries to establish the circumstances of the incident and find those responsible.
“I am appealing for anybody who saw what happened, or anyone with information about the incident, to get in touch. Any information you have, no matter how small it might seem, could be vital to the investigation.
“If you can help, please call the non-emergency number 101 or email me at john.cooper@herts.police.uk quoting crime reference 41/44419/21.
“We believe that the incident may have been recorded on camera and shared on social media. I would therefore like to remind the public that this may jeopardise legal proceedings and they should refrain from sharing any such video.”
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules here