A 10-year-old boy has been arrested over the sexual assault of a 23-year-old Italian tourist in Cairns.

The woman was walking past the child in the city's CBD at around 10am on Wednesday when he attacked her, police alleged. 

'The lady has called for police assistance and at that time she was set upon by a number of other juveniles,' Detective Inspector Kevin Goan said.

Officers arrested the boy as well as three others. 

A 10-year-old boy has been arrested over the sexual assault of a 23-year-old Italian tourist in Cairns (file picture)

The woman suffered bruises and scratches to her face but was not taken to hospital. 

'The young age of the offenders is abhorrent as far as I'm concerned,' Det Insp Goan said.

'People are entitled to go about their business in our community (and) for that to occur would have been a traumatic experience for the young lady. This is a sad indictment that she will take away from her experience in Cairns.'