An armed man was arrested on Sunday at the entrance of Buckingham Palace, the London police said, ruling out the possibility it was terror related.
Police said the 38-year-old was carrying a Taser stun gun, adding that he remains in custody.
"It's not terrorism related. It's just a single standalone incident," a police spokeswoman told AFP. She provided no other details.
The royal family members are currently on their summer break, with Queen Elizabeth II and Prince Philip staying at Balmoral Castle in Scotland.
Police said the incident occurred outside the visitors entrance, where the man was detained by security staff.
London remains on a heighten state of alert due to periodic terror-related incidents.
Most recently, in August, a terror suspect crashed his car into the security barriers surrounding the Houses of Parliament.
A Sudanese-born 29-year-old has been charged with two counts of attempted murder.
Get the latest election news, live updates and election schedule for Lok Sabha Elections 2019 on ndtv.com/elections. Like us on Facebook or follow us on Twitter and Instagram for updates from each of the 543 parliamentary seats for the 2019 Indian general elections. Election results will be out on May 23.