Shoppers watched in horror as a masked man walked into a busy Tesco Express store brandishing what appeared to be a handgun.
Three offenders walked into the shop in Hobs Moat Road, in Solihull, at 10.14am and entered the room where the safe for the store's cash machine was located, passing customers and staff on the way.
The men threatened a security guard who was making a delivery of cash with the gun, forcing him to hand over cash. They then made off with what was described as a " substantial" sum of money.
Detectives investigating the robbery said there were a number of people in the shop who saw the offenders with a gun.
"We have taken statements from some of them who were in the shop who said they were very scared," said Detective Sergeant Bill Lorne, of West Midlands Police. "There were a lot of people around at the time of the robbery.
"The guard was threatened but fortunately nobody was injured."
The men arrived and made their escape in a light blue four-door Alfa Romeo car. One man remained inside the vehicle while the robbery took place.
Det Sgt Lorne said the offenders could have waited outside for the security van to arrive or followed it to the Tesco store.
Afterwards, the men were seen driving towards Hobs Meadow Road. They were last seen in Richmond Road going towards Barn Lane. "We would appeal to anyone who saw the offenders or their car to contact the police," added Det Sgt Lorne.
Anyone with information should call 0845 113 5000 or Crimestoppers on 0800 555 111.