A fully-working Second World War sub-machine gun was found in a plastic bag in a residential street.
The M3 automatic weapon was recovered from Falcon Hey, Fazakerley , at around 5pm yesterday during an open land search.
Merseyside Police said an empty magazine was found lying on the ground nearby.
Forensic officers were witnessed examining the area last night but the scene has since been stood down.
The M3, capable of firing at a rate of 450 bullets per minute, was commissioned by the United States Army in 1942 and used towards the end of the Second World War and more heavily in the Korean War.
Detective Inspector Sean Kelly-Martland said: "This initially appears to be a viable firearm with the potential to cause serious fear and harm in our communities.
"We will be carrying out forensic enquiries to establish who has handled this weapons and if it has been used previously, either in Merseyside or elsewhere.
"Either way, a potentially dangerous weapon has been taken out of the hands of those who look to be involved in serious and organised crime.
"Open land searches are one of many tactics we carry out to remove dangerous weapons from the streets.
"If you know where any firearms or other weapons are being stored where you are, let us know and we can continue to take such positive action and keep the communities safe."
