Three supposed animal rights campaigners have been arrested on suspicion of conspiracy to blackmail.
A 50-year-old woman was detained in Croydon, south London, while a man and a woman aged 25 were picked up in Amsterdam. The suspects are believed to be animal rights activists.
Although the details of the nature of the blackmail claim have not been revealed, the arrests are understood to have followed an extensive Europe-wide investigation into animal rights extremism.
Officers are searching the suspects' homes as well as small businesses in London and the Netherlands.
The investigation was led by the Metropolitan Police Counter Terrorism Command and the National Domestic Extremism Unit in partnership with the Dutch National Crime Squad.