Police are questioning three youths - aged 15, 17 and 18 - on suspicion of robbery at a police station in north London.
The motive for the attack is unclear at this stage.
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A lamp in the tower which houses Big Ben is to be switched off for the first time in more than 70 years.
Sir Bruce entertained audiences for more than seven decades.
Work in the tower will also pause over the bank holiday weekend.