Queen Elizabeth II. Photo / AP
British police arrested a man armed with a weapon on the grounds of Windsor Castle on Christmas Day.
Thames Valley police said the security breach took place at 8.30am (British time).
The 19-year-old from Southampton is in custody.
"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 Superintendent 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 were informed.
Queen Elizabeth II is spending the Christmas holidays at Windsor, having decided not to stay at Sandringham, the royal estate in eastern England where she traditionally spends the festive season with family.
Reporting from the Mirror suggests the "offensive weapon" the suspect was allegedly carrying was a crossbow.
The publication cites "security sources" who say the man was spotted on CCTV "carrying what appeared to be a crossbow".
The source added: "There were real concerns the situation could have massively escalated having such a dangerous weapon to deal with.
"When word got out that he had been apprehended, everyone breathed a huge sigh of relief – but it's obviously tremendously concerning."
The investigation into the incident is "ongoing", Mears said.
The Queen was joined by some members of her family, including Prince Charles and his wife, the Duchess of Cornwall for Christmas celebrations at Windsor Castle, where she has remained throughout most of the pandemic.
The monarch delivered her annual Christmas message to the people of the UK and the Commonwealth.
Saying she understood the difficulty of spending the holiday season "with one familiar laugh missing," the monarch delivered her address beside a framed photograph of her arm-in-arm with Prince Philip, who died in April at age 99.
On her right shoulder was the same sapphire chrysanthemum brooch she wore in the photo — a glittering statement pin that she also wore as a newlywed.
Several Assad allies, including Maher and Mamlouk, sought refuge in Russia and Lebanon.