Five men with balaclavas, two crowbars and a sledgehammer left a jewellers on bicycles, police have said.
Officers received a call from a member of the public at 11.08pm yesterday to report a group of five men breaking into Ernest Jones in East Street, Chichester.
A member of the public followed the group but lost sight of them in Little London.
It is not known if anything was taken.
Detective sergeant Alison Hutchings said: “A couple of alert members of the public were able to quickly inform us about the break-in, allowing our officers to quickly attend the scene.
“The brave member of the public who followed the group was threatened by the men but luckily, was not harmed.
“The suspects were wearing balaclavas over their faces and we are appealing to anyone who may have seen the group fleeing the scene to get in contact with us immediately.”
If you witnessed the incident or have any information please report online or call 101 quoting reference 170 of 23/08.
Alternatively, you can visit the Crimestoppers website or contact the independent charity anonymously on 0800 555 111.