Marcus Powell spoke to a local resident who saw a police officer being attacked by the dog in East London.
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One of the teenagers was left so badly burnt doctors say he may lose his eyesight.
Jame Murray's challenge is to deliver Mr Khan's manifesto pledge to build more than 50,000 new homes a year - half of them affordable.
Ryan Lee was removed from the finishers' list because he was not picked up by the 5k checkpoint.