A man armed with a weapon was arrested in the grounds of Windsor Castle as Queen Elizabeth II celebrated Christmas there.
Thames Valley police said the security breach took place at 8.30am on Saturday.
A 19-year-old man from Southampton has been arrested on suspicion of breach or trespass of a protected site and possession of an offensive weapon, detectives say.
Newspaper reports claim the arrest came after security controllers monitoring CCTV spotted a man carrying what appeared to be a crossbow.
“The man has been arrested on suspicion of breach or trespass of a protected site and possession of an offensive weapon. He remains in custody at this time,” Thames Valley Police Supt Rebecca Mears said.
“We can confirm security processes were triggered within moments of the man entering the grounds and he did not enter any buildings.”
Members of the royal family have been informed.
Queen Elizabeth II usually spends the festive season with family at Sandringham, the royal estate in eastern England.
She is staying at Windsor Castle this year because of the pandemic.
