A hooded man threatened staff with a gun at a Bootle bookies before escaping with cash.
Officers were called to Betfred on Hawthorne Road to reports of a man having entered the premises with a firearm.
After making threats to staff, he took a quantity of cash and made off on a pedal cycle.
A number of customers were also in the bookies at the time.
Nobody was injured although staff and witnesses were reported to be "shocked", and are providing statements. Witness, CCTV and forensic enquiries are now being carried out.
The suspect is described as white, aged 20-25, with a neck warmer covering his face, a hood up and a dark tracksuit, riding a pedal cycle.
Detective Inspector Paul Gillies of Merseyside Police said: "For someone to use a firearm to threaten people while they carry out their business is despicable and we are carrying out extensive enquiries this evening.
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"The staff and witnesses present were understandably shocked to see such an incident.
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"If you were also in or around the bookmakers at the time and saw the incident or someone matching the suspect's description riding off, come forward if you haven’t already done so.
"Your description, CCTV, dashcam or other information could be the key to finding the person responsible and removing them from the community.
"We work closely with all businesses to ensure they are as safe as possible and will being doing all in our powers to bring this offender to justice."
You can pass information to police via the social media desk @MerPolCC on Twitter or 'Merseyside Police Contact Centre' on Facebook, or call the independent charity @CrimestoppersUK anonymously on 0800 555 111, with reference 20000605068.
You can also contact them via their online form here.