Surrey Police said that the 54-year-old man was arrested in Leatherhead, about three-quarters of the way round the 250-kilometre route.
A police spokeswoman said the spectator had strayed "too close for comfort" to the cyclists. She spoke on customary condition of anonymity in line with policy.
Police detained the man at the scene, but confirmed that the event was not disrupted by his actions.
They warned the public not to encroach onto the course when the women's race takes place Sunday.
Police superintendent Jerry Westerman said several hundred thousand people lined the route of the race, won by Kazakstan's Alexander Vinokourov.
"The atmosphere around the course has been absolutely fantastic," Westerman said.