A taxi driver was threatened with a gun while picking up a fare in the early hours of Saturday morning.
At 3am the victim stopped at Lloyds in Coffee Hall, close to the Church of Our Lady Lourdes, to pick up a passenger when he was approached by three men.
They opened the car doors, demanded money and then assaulted the driver. He was also threatened with what is believed to be a handgun.
The offenders then ran off along the footpath behind Rochfords in Coffee hall, in the direction of the V6 Grafton Street.
Cash, a white Samsung Galaxy Fame PDA and a black Samsung ACE 2 mobile phone were stolen.
The victim, who is in his 40s, sustained bruising to his face but did not require hospital treatment.
The offenders were all men and they were dressed in dark clothing with hoods and scarves covering their faces.
Det Con Greg Hughes from Milton Keynes Force CID said: “The offenders may have been disturbed by a member of the public who was seen by the victim to be walking from the direction of Leadenhall.
“We are keen to trace this person as they may have information which could help with our investigation.
“This was a frightening ordeal for the victim who was just going about his business when he was attacked.
“Thames Valley Police will not tolerate violent crime in our community and officers are doing everything they can to trace the offenders in this incident so they can be brought to justice.
“I would urge anyone with any information to contact me via the 101 number or call Crimestoppers anonymously.”