Two men were attacked in the incident - and police are hoping residents can identify five people they want to speak to.
The first victim, a man in his 50s, was walking past Middletons Steak House, in Bridge Street, at about 8.30pm, when he bumped into a group of men walking towards him. The group shouted at the man and as he tried to walk away he was a punched to the ground and stamped on.
A second man, also aged in his 50s, who was in the restaurant having a meal with his family, witnessed the attack and intervened but was struck to the head with a skateboard by one of the group. He suffered a serious head injury and was taken to Peterborough City Hospital.
The suspects left the area, heading in the direction of Town Bridge.
PC Jon Coupland said: “This was an unprovoked attack where a victim has suffered a nasty head injury in the process of trying to prevent another person from being further assaulted. He was out for a meal with his family who witnessed what happened. We are keen to speak to anyone who was there at the time or recognises the men in these images.”
Anyone with information should call police on 101 quoting 35/90296/21 or visit www.cambs.police.uk/ report Alternatively, contact Crimestoppers, anonymously, on 0800 555111 or via www.crimestoppers-uk.org.