Police have said the alleged attack of a young Muslim woman came completely unprovoked as she walked down a busy city street.

An unknown man allegedly punched a 21-year-old Muslim woman, from Dandenong, outside the State Library of Victoria about 1.10pm (AEDT) on Thursday, causing her to fall to the ground.

Police said the offender struck the woman from behind as they passed each other in opposite directions on Swanston Street.

Fairfax Media reported on Friday morning a 16-year-old Muslim boy, who witnessed the incident, allegedly had a knife thrust in his face as he attempted to help the victim. They also reported the woman had a hijab ripped from her head.

• Joe Hockey’s wife evicts his mate
• Third-largest hole ever opens in ozone layer
• Lesbians ‘arrested for kissing’

But police said a weapon was not involved in the alleged attack, nor were they aware of the woman’s hijab being removed.

Speaking to the media on Friday, Acting Sergent Sean Toohey said they did not believe the incident was racially motivated, contrary to prior reports.

“We have no information to suggest what motivated it at this stage the victim said nothing said to her she just felt the strike to the head,” he said.

“She is pretty well apart from still being in shock, physically she is okay.”

Paramedics treated the woman at the scene.

The offender was described as being Caucasian, 183cm tall and approximately 45-years old with a slim build, short light-coloured hair and a beard with a tattoo on his upper right arm. He was shirtless at the time of the incident. 

The father of the 16-year-old boy told The Age his son was “very upset” by the alleged attack.

“My son told him: ‘What are you doing?’. The man was swearing. Then he pulled a knife out on my son. He said: ‘If you go near me, I’m going to hurt you’,” he said.

The incident was also referred to Islamic advocacy group, Islamophobia Register.

The group said they had spoken to the boy on Friday and he was “still ‘shaken up’ by the incident”.