A 78-year-old dog walker was left with a fractured jaw and a bleed to the brain after being attacked in Wardown Park on Saturday.
Police are looking for three Asian teenagers who were seen in the park before and after the assault 3pm.
The man was wearing a mustard coloured jacket and had been walking his small dog around the park near the lake.
One of the teenagers is described as a dark skinned Asian male, about 16 years old, skinny with long hair on top and short on the side, about five feet seven inches.
He was with two other Asian youths of around 15-17 years old.
It is believed that they may have been in the park prior to the assault by as much as an hour and may have been by the ice cream stand.
Detective Sergeant Tom Hamm said: “The victim is in a stable condition in hospital having suffered a fractured jaw and a bleed to the brain.
“This was a very serious assault and we are asking anybody who was in Wardown Park between 1pm and 4pm to contact us if they saw this incident or the three Asian youths in the park before or after the attack took place.”
Anyone with information relating to this incident can contact DS Hamm, in confidence, at Bedfordshire Police on 101, or text information to 07786 200011.
Alternatively you can contact the independent crime fighting charity, Crimestoppers, anonymously, on 0800 555 111.