A man was slashed across the neck as his attacker left him bleeding in the street.
Emergency services were called to Albion Street, in St Helens, shortly after 8pm on New Year's Eve.
The victim, a man in his 20s, was slashed close to the library before his attacker ran from the scene.
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According to Merseyside Police, the man's attacked is described as being a white man, in his 30s with short hair, and wearing a grey tracksuit.
A short time later the victim was rushed to hospital for treatment where his condition is described as serious but stable.
CCTV, witness and forensic enquiries are ongoing at the scene and surrounding streets.
Detective Inspector Tony O’Brien said: “Thankfully the victim is getting the care he needs in hospital, but this nasty assault has left him with a very serious injury which could so easily have been catastrophic.
“Our enquiries are at a very early stage and we are keeping an open mind as to the motive of this violent attack.
“Carrying any dangerous weapon can destroy lives, and we will respond to any information on those who store, carry or use them in our communities.”
Det Insp O’Brien added: “Our officers carry out work 365 days a year to rid our streets of knives and weapons, and bring offenders to justice.
"We conduct open land searches, stop searches, targeted operations and high visibility patrols, as well as working closely with our communities, visiting schools and involving safeguarding partners.
“Vital to these efforts is information from the public on where knives and weapons are stored and who is carrying them, so tell us what you know and we will do the rest.”
Anyone with information, images or footage to assist the investigation can contact us via @MerPolCC on Twitter or 'Merseyside Police Contact Centre' on Facebook, with reference 22000001132. Always call 999 if a crime is in progress.
You can also contact the independent charity Crimestoppers anonymously on 0800 555 111 or via their online form at: https://crimestoppers-uk.org/give-information/give-information.
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