Staff attorney Rachel Meeropol says the 2006 law has left activists afraid to participate in public protests out of fear they will be prosecuted.
"There are many terms in the law that are not defined, and because of that protesters don't have notice that certain conduct is going to violate the statute and what conduct is protected by the First Amendment," Meeropol said.
"Some of my clients want to engage in simple public protests - perhaps in front of a fur store - to change public opinion about fur," she said. "But they feel restricted from engaging in that clearly lawful activity because under the plain language of the law, if that protest is successful in convincing consumers not to shop at that fur store, they could be charged as terrorists."
The law can be used to prosecute someone for damaging or interfering with the operations of an "animal enterprise" when a person "intentionally damages or causes the loss of any real or personal property used by an animal enterprise" or a business connected to an animal enterprise.
Meeropol said courts have interpreted "personal property" to include a loss of profits for the business.
The law also can be used to prosecute anyone who "intentionally places a person in reasonable fear of death or serious bodily injury" through threats, vandalism, harassment or intimidation.
U.S. Attorney General Eric Holder is the only defendant named in the lawsuit. A Justice Department representative did not immediately return a call seeking comment.
The lawsuit lists only seven people who have been prosecuted under the law. Meeropol acknowledged that it has not been used often, but she said even one high-profile prosecution could instill fear among animal rights activists.
Ryan Shapiro, a longtime animal rights activist from Cambridge who is one of the plaintiffs, said he no longer conducts undercover filmed investigations of animal treatment on factory farms because he is concerned about possible prosecution.
"One of the ways the Animal Enterprise Terrorism Act silences free speech is that if one obtains that footage and then brings that footage to the public about how animals are suffering on factory farms, it might affect the profits of that farm," Shapiro said. "As a result, simply bringing that information to the public and trying to educate individuals is now prosecutable as a terrorist act under the law."