Police said late Wednesday night that the 22-year-old man was arrested earlier that morning by specialist royal police officers.
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Palace officials declined to say whether the queen was told about the incident.
According to police, the alleged intruder wasn’t carrying a weapon, and the incident is not seen as related to any extremist threat.
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He was held on suspicion of trespassing at around 2 a.m. on Wednesday by specialist royal police officers, the Metropolitan police said.
“The 22-year-old man was arrested at approximately 2 a.m. on Wednesday, 10 July, by officers from the Met’s royalty and specialist protection command after he climbed over the front gates at Buckingham Palace,” a Metropolitan spokeswoman said.
Police declined to disclose whether there would be a review of security after the incident.
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The suspect is in police custody. He hasn’t been charged or identified.
—With files from the Associated Press