There was a woman in her 60's who was standing in the cashier's box when a man armed with what appeared to be a knife came in.
"He must have threatened the woman and brought along cigarettes and an unknown amount of money," says Stefan Dangardt, press spokesman for the Bergslagen Police Region.
The woman then locked herself in until police arrived at the scene. She is said to have been shocked but not physically injured.
Police have searched with several patrols for the robber, who is reported to be a young man, but has not yet arrested anyone. The search will continue during the day.
Earlier on Wednesday, an elderly woman was subjected to a robbery attempt in an elevator in her apartment building in the Bro area. The police do not rule out that the events have a connection and it may be the same offender.