A mental health patient given home leave after being declared “no risk” was later found by police brandishing a Samurai sword in the street.
Police were forced to twice Taser the man after he drew the sword from its sheath as he approached the officers in Plymouth.
Health officials said the man had been granted home leave from Lee Mill Hospital in Ivybridge, Devon, following a clinical assessment. An investigation has now been launched.
Officers from Devon and Cornwall Police were confronted by the man in Devonport Road following calls from members of the public.
The force said officers were authorised to use Tasers to detain the man who "posed a substantial threat".
A spokeswoman said the man had refused instructions to drop the sword and officers used a Taser as he reached Milehouse Road.
But the first Taser failed to stop the man who continued to approach the officers and removed the sword from its sheath, upon which they used the Taser for a second time.
A witness, who wanted to remain anonymous, told The Plymouth Herald she saw a man "swinging" a sword and cutting through Taser wires attached to him before he was detained.
The man was returned to Lee Mill Hospital following the incident at around 9am on Sunday.
The hospital, run by Plymouth Community Healthcare, offers services to people with mental health, learning disability or substance misuse.
Liz Cooney, director of governance and deputy chief executive of Plymouth Community Healthcare, said: "Plymouth Community Healthcare can confirm that the individual involved had been granted home leave from the Lee Mill unit following a clinical assessment.
"At the time of granting leave this individual was not deemed to be a risk to themselves or others.
"No persons were harmed during this incident and the individual involved has now been returned to the care of staff at Lee Mill.
"Plymouth Community Healthcare takes all such incidents very seriously and a full investigation has commenced into the incident."
A spokesman for Devon and Cornwall Police said inquiries were continuing into the incident.
A 36-year-old man arrested on suspicion of affray has been sectioned under the Mental Health Act.