Seven teenage girls allegedly approached two woman walking in Wanneroo last night before one of the girls smashed one woman in the head with a glass.
A police spokeswoman said that two women aged 25 and 18 years of age were walking along Bamboore Crescent in Wanneroo about 10.45pm when they were approached by a group of approximately seven female teenagers.
"One teenager asked one of the women for a cigarette, and when she said they didn’t smoke the two women turned to walk away," police said.
Police allege one of the teenagers grabbed the 25-year-old woman by her hair, causing her to fall backwards on to the ground before hitting her on the head with a glass, shattering it.
"As the woman’s 18 year old friend tried to help her, several other teenagers grabbed her and the scuffle continued," police said.
"The group of teenagers ran off and were last seen heading east near the pedestrian laneway on to Belgrade Road."
The teenagers are described as being 16 and 17 years of age and fair skinned.
Four teenagers have been taken into custody.