A woman was removed from a quayside during a visit by the Duke and Duchess of Cornwall.
The anti-Brexit protestor was moved on by police when she tried to push past a cordon in St Mary's.
Police said the visitor to the island was given a warning under section five of the public order act.
Prince Charles and Camilla are in the Isles of Scilly to open a new quayside as part of their annual trip to the South West of England.
Local businesses hope the quay will attract more visitors to the area.
Tourism accounts for more than 85% of its economy.
During their visit to the South West, the royal couple have visited the Duchy of Cornwall Nursery in Lostwithiel, attended an ocean plastics solution day in St Agnes and named Fowey Gig Club's new rowing boat.
On Wednesday they are due to visit a community shop in St Tudy and a food market in Honiton.
Camilla is also celebrating her 71st birthday in the islands on Tuesday.