A 14-year-old boy has lost his hand after picking up a device which exploded.
The boy, a young army cadet, was at the Territorial Army base in White City, west London.
It is believed the boy picked the device up from outside and took it inside the TA building, on South Africa Road, where it went off.
Chief Superintendent Anthony Wills of the Metropolitan Police said: "Police were called by the London Fire Brigade. We attended the scene with the London Ambulance Service and a young lad aged 14 suffered quite serious injuries as a result of the explosion."
He added: "The explosion took place inside the Territorial Army establishment. It was an explosive device of some kind."
There had been no terrorist warning, he said.
The teenager was taken to the Chelsea and Westminster Hospital in Chelsea where a spokeswoman said: "We can confirm that a 14-year-old boy was admitted to the hospital at approximately 7.45pm following an explosion at White City.
"His injuries are currently being investigated though I can confirm that his hand was lost in the explosion," she added.
The TA centre is next to the BBC Television Centre in White City and on the same road as Queen's Park Rangers' football club ground, Loftus Road.
South Africa Road runs from the main A40 Wood Lane into the suburbs of White City and Shepherds Bush. White City tube station, which is adjacent to South Africa Road, was closed for a while, but later reopened.