Although all are American, one of the group was a key player in the movement in Britain and controlled Stop Huntingdon Animal Cruelty (Shac) while its British leaders were in prison.

The US arm of Shac itself was convicted, along with the six activists, of inciting violence and terrorism in an attempt to shut down Huntingdon Life Sciences (HLS), Britain’s largest animal testing facility, which has a branch in New Jersey.

The verdict is a victory for the American authorities, which have become increasingly concerned about the growing severity of attacks. It comes months after the FBI said that animal rights extremists posed the greatest domestic terrorism threat in America.

The conviction is thought to be the first achieved under the animal enterprise terrorism legislation, which is similar to “economic sabotage” laws that were introduced in Britain last summer, but was designed specifically to combat animal rights extremism. The offences were similar to those suffered by British victims of the campaign — Shac used its website to incite threats, harassment and vandalism.

Many people received threats after personal information about employees of HLS and related companies — some of which had only a loose connection with the laboratory — was posted on the internet. This included the schools attended by their children.

Demonstrations were held outside homes, during which protesters waved photos of mutilated animals and shouted abuse.

One woman received an e-mail threatening to cut open her seven-year-old son and fill him with poison. A man was showered with glass when the windows of his home were smashed by activists who overturned his wife’s car.

Shac sent thousands of e-mails and faxes to targeted companies to disrupt their systems. Some firms bowed to pressure and severed their ties with HLS.

The defendants were accused of inciting others to commit harassment. They maintained that their actions constituted free speech.

Joshua Harper, 31, Lauren Gazzola, 26, Jacob Conroy, 30, Andrew Stepanian, 27, Darius Fullmer, 29, and Kevin Kjonaas, 28, were convicted of animal enterprise terrorism after the three-week trial in New Jersey. Kjonaas, Gazzola and Conroy were convicted of multiple counts of conspiring and committing interstate stalking and of telephone harassment. Harper was also found guilty of telephone harassment.

Those convicted of all three offences face a maximum fourteen years in prison and substantial fines when sentenced in June. Shac as an organisation was also convicted and its American website was taken down.

Kjonaas, the former president of Shac America, came to Britain in 2001 to work alongside British activists. He was sent back to the US after two years when police realised his visa had expired.

In an interview with an internet magazine, “Kevin Jonas” said: “I spent a year in England working full-time on animal rights campaigns and there really cut my teeth on some ‘true grit’ activism.”

The convictions were welcomed both sides of the Atlantic. An HLS spokesman said: “This is a victory for democracy, biomedical research and patients.”

The US Attorney’s Office said: “The convictions defeat the argument that these so-called activists were acting within their rights. (They are) thugs who went far beyond protected speech and lawful protest to engage in and incite intimidation, harassment and violence.” It was the second trial faced by the group. The first ended in a mistrial after Kjonaas’s lawyer became ill.

• One of Shac’s founder members was jailed yesterday for breaching an anti social behaviour order. Heather Nicholson, who also uses the surnames Avery, Barwick and James, was given a four-month prison sentence by Oxford Crown Court.

  She admitted breaching an ASBO imposed in January, which did not allow her to go near sites at Oxford University, HLS or the pharmaceutical company Phytopharm, or contact employees or their families.

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