Andrew Marshall, 44, from Nottingham, was searched by police as he walked through The Mall area around St James’s Palace at night.
Officers used a set of keys found in his possession to access a vehicle in the car park of the palace.
It has been reported that it was there where they found a crossbow, 14 bolts and a large knife as well as a pair of latex gloves.
The 44-year-old, who is believed to have mental health problems, was arrested and charged with two counts of weapon possession in a public place.
Marshall is yet to enter a plea and was yesterday bailed by Westminster magistrates to appear at Southwark Crown Court on March 19, on the condition that he will remain in hospital until that date.