A spate of bike thefts occurred as the Tour De France raced through Woodford Green on Monday.
As crowds flocked to view Chris Froome and the other riders head through the borough, thives were at work.
Norma Hudson, 85, of Palmerston Road, said her grandson’s £400 bike was stolen "right under the nose of a policeman" while spectators were distracted by the race.
The bike, which Jack Hudson had borrowed from his father especially to watch the event, was chained to a lamppost in Johnston Road.
Mrs Hudson said: “I was flabbergasted and when I heard his bike was stolen, it ruined my day.
“What was supposed to be a joyful occasion was totally ruined by these miscreants.
“He was so upset and he was frightened to tell his father.”
Her grandson had been playing football with his three friends before going to watch the race at Woodford Green, locking all their bikes all together.
It had been locked for "mere minutes" before the thief struck as the race went past.
When Mr Hudson went to the police, another man was reporting the theft of his £1000 bike at the same time.
Mrs Hudson said: “If I was younger I would find whoever did this and beat them up.
“At the very least I hope they fall off their bike.”
The Guardian has asked police how many other bikes were stolen during the event.