The 13-year-old victim was walking along Fratton Road between 5pm and 5.20pm when he was approached from behind by three boys.
He was then attacked outside of Barrells Funeral Directors, near the junction with St Mary's Road, leaving him with injuries to his neck, nose and elbow.
An investigation into the incident is ongoing.
A police spokeswoman said: ‘We believe that a couple of people who were driving past at the time of the incident got out of their cars to help the victim following the assault.
‘We are very keen to identify and speak to these people as part of our enquiries. We’d also like to hear from anyone else who was in the area at the time and saw what happened, the moments leading up to it or the aftermath.’
Call 101 quoting reference 44220242966 with information.