Officers from Thames Valley Police (TVP) and the Metropolitan Police responded to a security breach on Saturday at around 8.30am within the grounds of the castle in Berkshire where the Queen is in residence.
A 19-year-old man from Southampton was arrested and following a search by officers responding from both Thames Valley Police and the Met, a crossbow was recovered.
The man was arrested on suspicion of breach or trespass of a protected site and possession of an offensive weapon.
In a statement, the Met Police said: “The man was taken into custody and has undergone a mental health assessment – he has since been sectioned under the Mental Health Act and remains in the care of medical professionals.
“Inquiries into the full circumstances of this incident are being progressed by Metropolitan Police Specialist Operations.”
The force confirmed security processes were triggered within moments of the man entering the grounds and he did not access any buildings.