Spectators at the Tour de France have been told by organisers not to run alongside riders or step into the road. A short time ago, a spectator was knocked over after getting too close to the action while trying to take a picture.
Spectators began to gather along the route of the race hours before the cyclists were due to speed past.
It is understood the Mayor's trip to the US was "long-planned".
Significant travel delays are expected today as the Tour de France arrives in London.