The thief, armed with a firearm, stole a large quantity of cash from a newsagent in Stevenage.
No one was hurt during the incident.
Hertfordshire Police used a helicopter to search for the offender, who was thought to be heading towards the town centre.
They have also stationed “a large number of officers” in the area, including armed police.
The incident took place at McMcolls Newsagent at around 7.35pm yesterday.
A spokesperson for the force said: “A person – believed to be a white male – entered the store and threatened assistants with what is believed to be some sort of firearm.
“The offender then left the store with a quantity of cash, heading in the direction of the town centre.
“He is believed to be wearing a grey hoodie with writing on the sleeves, black trainers and black gloves and had an Adidas rucksack on his back.
“Whilst in the store he was wearing a Halloween style mask.
“No one was hurt during the incident, but a large number of officers are currently in the area, including the armed policing unit, and the police helicopter is also helping with the search for the offender.”
“Anyone with any information which may help the investigation is asked to contact police immediately.”
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