Three people have been arrested in Amsterdam and London after a Europe-wide probe into blackmail linked to animal rights extremism.
A 50-year-old woman was held in Croydon, south London, on suspicion of conspiracy to blackmail.
She was arrested by officers from Scotland Yard's Counter Terrorism command, who led the investigation.
Two other people were arrested in the Netherlands.
A man and a woman, both 25, were arrested at a residential address in Amsterdam under a European Arrest Warrant, the Metropolitan Police said.
Officers are searching the home in Croydon where the 50-year-old was arrested, and a business address in central London, while Dutch counterparts are searching a home and business in Amsterdam.
The alleged plot focused on a target in the UK but Scotland Yard would not reveal whether it was an individual or a business.
Two of the people arrested are British and one is Dutch, the force said.
Extradition proceedings will start in the Netherlands to try to bring the arrested pair back to this country.