Two robbers held terrified staff at gunpoint before seizing cash from Thorpe Edge Post Office.
The raiders, who both wore balaclava helmets, burst into the post office on Idlethorpe Way.
One man smashed the counter glass and demanded money while brandishing a hand gun.
But a female member of staff calmly walked into the back office during the raid to phone 999. She hung up, concerned about the safety of her colleagues, when she realised the robbers might hear her talking to the police.
It is believed the robbers took a considerable sum of money in the raid at about 1pm yesterday.
They escaped in a red Vauxhall car with the registration starting F683 X. It was parked on Crag View overlooking the post office.
The car sped off along Idlethorpe Way before turning right into Averingcliffe.
Det Insp John Chambers, of Eccleshill CID, said the robbery was a particularly vicious attack and appealed for any witnesses to come forward.
Anne Short, owner of nearby Anne's Pantry, said she understood one of the robbers had held a gun to the head of a female post office worker in her mid-20s.
She said: "She was extremely shaken and they are not expecting her back into work this week. I know them down there and they are very shaken. I've been here for seven years and none of the shops have suffered anything like this. Customers are on edge today.''
Another shop owner, who did not want to be named, said he was held up by gunmen while working at a petrol station in Manningham Lane five weeks ago. "It's terrifying when you have a gun pointed at you. You just hand over the money,'' he said.
"They're very brave in the Post Office to be working today."
One of the robbers is described as white, 5ft 6ins tall, aged between 16 and 18-years-old, with short, fair hair.
Witnesses say the youth was very thin and had prominent cheek bones.
The second robber was described as being slightly taller and broader.
A spokesman for Post Office Counters, said: "We always take the safety of our staff very seriously and we do give them training in how to handle situations like these.
"The staff involved in the incident are very upset."
Anyone with information should contact Eccleshill police on (01274) 376082 or Crimestoppers 0800 555111.
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