agent provocateur

provokaattori A person who secretly disrupts a group's activities from within the group; an instigator, troublemaker.
1990, (w), The Great Game, Folio Society 2010, p. 50:

Suspecting that the man was an agent provocateur sent by the Prince, Pottinger told him that regrettably he had neither the knowledge nor the authority to instruct him in this or any other religion.

The criminalisation of protest has been aided by the director of public prosecution's recent guidelines on prosecuting protesters, which fail to account for the role of the police in prompting violence at demonstrations through tactics such as kettling, the use of batons, agents provocateurs and undercover policing.