Two masked men robbed the Post Office in Battlers Green Drive, Radlett on Tuesday.
The incident happened at 5pm. The two offenders went into the post office and took money from the counter.
Whilst taking the money, one of them pointed what it believed to be a handgun, which was hidden under his jacket, and made threats.
They demanded the safe to be opened but left before this was complete.
A 47-year-old man who runs the Post Office was present at the time, along with his two young children, a shop assistant and a customer.
No-one was injured during the robbery.
Police are appealing for witnesses and information.
Detective Sergeant Alex Warwick said: "This was a terrifying ordeal for all involved and a significant amount of money was taken.
"The two men had parked in nearby Orchard Close and walked or ran to and from the shop. They had their faces covered and were wearing high visibility jackets and would have stood out.
"I would appeal to anyone who was in the area at the time and witnessed the offenders coming or going, or anyone who has any other information."
Anyone with any information should contact the police on 101 or call Crimestoppers anonymously on 0800 555 111.
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