The man walked into the betting office in Cricklewood Broadway on Monday morning. He demanded money but was refused by staff.
He threw a suspicious device into the secure area and produced a remote control, which made a beeping noise. He then took out a knife before escaping.
The item was found to be an improvised explosive device and was made safe.
Nothing was stolen in the incident, though one member of staff suffered an arm injury during the raider's escape. The area was cordoned off as a precaution.
Police said: "The suspect is described as of Mediterranean appearance, aged between 32 and 40 and he is 5"6' tall and clean shaven.
"He is described as stocky and overweight. He has black hair and was wearing a long dark blue polo top and black sunglasses. He speaks English with a foreign accent."
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