The man, aged in his 20s, was walking his dog at a park next to Woodland Street, Fratton, at around 5.15pm when he walked past a group of 15 to 20 men next to several parked cars.
After a brief exchange, the group walked towards him and his dog ran away – at which point they pulled the man over and began kicking and punching him repeatedly in the face and torso.
It has been reported he was dragged to the kerb but managed to run away, and has been left with internal bruising and facial injuries.
The men have been described as white, aged between 20 and 40 and are said to have been speaking a foreign language, possibly Romanian, during the attack.
One particular man was also described as 5ft 10ins tall, of a large build and wearing a black tracksuit with white stripes on the arms and black trainers.
The group reportedly drove away from the scene in a charcoal grey BMW, a white Mercedes and a metallic red VW Golf.
DC Heather Wensley said: “This was an unprovoked attack in daylight hours while a man was just out walking his dog.
“We want to hear from anyone who was in the vicinity of St Mary’s Church on Sunday afternoon who saw this incident, or this group and the vehicles described.
“We would also like to hear from nearby residents who might have CCTV footage that could assist us in identifying those involved.”
Anyone with information can call us on 101, quoting reference number 44210157173. Alternatively, they can contact the independent charity Crimestoppers anonymously on 0800 555 111.