The Metropolitan Police says it's looking into whether the substance sprayed into the face of an officer may have been ammonia. He was attacked when he stopped a moped rider in Battersea yesterday afternoon.
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A Met Police officer was left red-faced after attempting to stop a cyclist riding along a dedicated cycle path.
The number of people allowed into official spectator areas will be cut by a fifth with just 100,000 tickets available.
The mayor was being challenged by Labour following his selection as the Tory candidate for the safe seat of Uxbridge and South Ruislip.