The Boston campaign culminated with the activists standing outside the businessman's house day and night, using a megaphone to scream chants like "what comes around, goes around ... burn his house to the ground," Massachusetts prosecutors said.

The defendants were linked to Stop Huntingdon Animal Cruelty, which has a following on both sides of the Atlantic and has campaigned to drive the British firm out of business by threatening its clients and business partners, prosecutors said.

Massachusetts prosecutors said the activists' goal in Boston was to intimidate the victim's employer, Marsh Inc., the world's No. 1 insurance broker, into refusing insurance brokerage business related to Huntingdon.

The employee had nothing to do with animal testing laboratories or of any of Marsh's insurance brokerage business related to them, prosecutors said.

Marsh Inc., a unit of Marsh & McLennan Cos. Inc. , said it was pleased by the response of law enforcement authorities to the activists' "illegal and harassing activities." The company declined to specify the nature of its relationship, if any, with Huntingdon.

The most serious charge facing the defendants was attempted extortion, which carries a maximum sentence of 15 years in state prison and a fine of up to $5,000.

Actions against Huntingdon, which has held about 70,000 animals for testing, escalated in 2001 when its managing director was beaten with a baseball bat.