A man has been arrested after a mother and a child were attacked outside the Victoria Police headquarters in Melbourne’s CBD.

Police have been told the woman was walking with her three children outside the police station on Spencer Street about 1pm when a man punched her 10-year-old son in the upper body.

Superintendent Zorka Dunstan said the man initially walked off but turned around and grabbed the mother, putting her in a headlock.

“He started to stab her repeatedly with a pair of scissors,” Superintendent Dunstan said.

She said the woman suffered injuries to her back and shoulders.

Superintendent Dunstan praised a passer-by who jumped onto the man and pulled him from the woman before two others stepped in to help restrain the accused.

“It’s heroic,” she said.

“Without a doubt without their intervention it could have been a lot worse so we are so grateful for their assistance.”

She said police quickly arrived on the scene and arrested a 27-year-old Sunshine West man.

Superintendent Dunstan said the parties did not know each other, and the motive for the attack was unknown.

“It’s completely random, we’re horrified by it.”A dark haired woman wearing a navy police cap and police uniform stares at the camera.

Superintendent Zorka Dunstan says police are horrified by the attack on the mother and her son. (ABC News)

The family was visiting Melbourne from Cowes on Phillip Island.

The mother and son were taken to hospital with non-life threatening injuries.

“We also have the two other kids who witnessed the attack,” Superintendent Dunstan said.

“Fortunately, they weren’t injured but obviously they have been impacted witnessing such an incident, especially occurring to their mother and brother.”

Superintendent Dunstan revealed she saw the incident from her office window.

“One of the boys who was 11 years old, he was crying … I gave him a hug because I was impacted just watching how he reacted.”

The arrested man is yet to be interviewed by police.

Superintendent Dunstan said the man was known to police in Victoria and Western Australia, and detectives would look at whether mental health was an issue.

An investigation into the incident is continuing.