A man was assaulted and 11 people were allegedly caught with drugs at the Groovin' The Moo music festival in Bunbury on Saturday.
A 34-year-old Perth man was knocked unconscious and treated for bruising and swelling to his temple, jaw and lip after being assaulted at the festival at about 7.10pm.
A man is helping police with their investigation and officers are calling for witnesses.
A police officer and a security guard received minor injuries after allegedly being assaulted at the festival.
A drug detection dog allegedly found drugs on 11 people at the festival and 22 people were charged with a range of offences including drug possession with the intent to sell or supply.
About 20,000 people attended the festival which is in its fifth year.
Anyone with information on the assault is asked to contact Crime Stoppers on 1800 333 000.