Three masked raiders, armed with a machete and a handgun, burst into a service station in Middleton and ran off with cash.
The robbery took place at about 8.45pm on Wednesday at the Hattont Service Station on Greengate.
When they ran into the station, they threatened staff and customers with the weapons, took cash, and ran off towards Mainway.
They were all wearing grey hooded tops and had their faces covered.
Detective Constable Nick Price said: "We do not believe that the offenders had a getaway car so we are appealing for witnesses who may have seen three men running from the service station.
"These thugs stormed the station and terrorised people inside. We are asking anyone with information to help us."
Anyone with information is asked to call police on 0161 856 1977 or call the independent charity Crimestoppers anonymously on 0800 555 111.
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