Tensions boiled over ahead of a second demonstration against the cliff at Whipsiderry being concreted on Thursday.
A scuffle broke out between a protester and security staff at the beach.
The contractor is permitted to work between 8am and 4pm but nearby residents say they were woken by a crane lifting down fencing onto the beach at 6.30am.
Residents say they have reported the alleged planning breach to Cornwall Council.
Eco warriors were also removed from the site by security on Tuesday and Wednesday.
More than 150 people gathered for the Save Whipsiderry Cliffs protest to highlight the “desecration” and “destruction” of the natural environment, wild bird and mammal habitat.
They waved various placards, which included ‘Save Our Wildlife,’ ‘No More Greed, Save Whipsiderry’ and ‘Save the Cliffs and Wildlife.’
Organisers called on people to help halt the cliff reinforcement works taking place, which are needed to enable seven luxury villas to be built on the clifftop above at the former Paradise Cove Hotel.
They want to disrupt the works until March 31 as that is the point at which the developer will have to stop work due to the bird nesting season.
The Save Whipsiderry Cliffs protest group hope that will buy them time to persuade the Marine Management Organisation (MMO) to withdraw the licence to carry out the cliff works.