Staff were threatened with a lump of concrete during a robbery at a convenience store in Fair Oak, police said.
Hampshire Constabulary have lauched an appeal for witnesses after the incident, at Welcome Stores on Fair Oak Road.
It happened at 8.45pm on Saturday evening.
A man threatened staff with the concrete, stealing around £300 from the till before fleeing.
READ MORE: Call 999 if you see missing woman last seen near Westquay
Two bystanders attempted to intervene and chased the suspect.
The thief, described as white, around 5ft 8in tall, and slim, was wearing dark jogging bottoms and a bright blue raincoat with the hood up.
A spokesperson for Hampshire Constabulary said: "We would like to speak to anyone who can identify this man, or the two members of the public who tried to help.
"Anyone with information or relevant CCTV or dash cam footage should call 101 quoting reference 44250160549."
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules hereComments are closed on this article