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Many of those detained in the unprecedented sweep were associated with Stop Huntingdon Animal Cruelty (SHAC), including leader Greg Avery. SHAC has led an eight-year campaign to shut down Huntingdon Life Sciences, an animal testing firm that moved to the United States from England after years of attacks including fire-bombings of vehicles and vandalism of Huntingdon employees' homes.

The raids took place across England, Scotland, Wales, Belgium and the Netherlands, although all the arrests were made in the United Kingdom. Cash, cell phones, computer equipment and documents were seized in the searches.

England long has been the epicenter of violent animal rights activism, although the activities of SHAC and other militant organizations have spread to the United States in recent years. Several notorious cases created a political storm in Britain in recent years, with public opinion increasingly turning against the activists. In 2005, Britain passed a tough new law targeting SHAC and similar groups.

Militants associated with SHAC and similar organizations increasingly have engaged in "secondary" and "tertiary" targeting, meaning they have moved from targeting Huntingdon employees and their families to singling out officials and their families at companies that work with or even merely insure Huntingdon. Some of these protests, many of which occurred at officials' homes, have been violent.

"In recent years, animal rights extremists have conducted sustained campaigns of harassment and intimidation against the animal research industry, seeking to achieve their object by creating a climate of fear," said Adrian Leppard, an English police official. "Although the vast majority of animal rights protesters campaign lawfully, a small minority seeks to force change through criminal action."