Armed police were called after a man sustained deep cuts to his head and leg.
Police were called to reports of an assault in Riverside Road, near Watford General Hospital, just after 3pm on Thursday.
A man in his 20s was found injured when police arrived, and he was taken to hospital. He has since been discharged.
Hertfordshire Constabulary says a number of people fled the area before they arrived and a search was carried out by police, including armed officers, while the helicopter hovered above.
One man was detained and a vehicle was recovered in nearby Jellicoe Road as part of the investigation.
Enquiries are ongoing.
• A 42-year-old man from Watford was initially arrested on suspicion of grievous bodily harm in connection with the incident, however he has been released with no further action.
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