Berkeley, California: In the hills above the University of California's Berkeley campus, nine protesters gathered in front of the home of a toxicology professor, their faces covered with scarves and hoods despite the warm spring weather.

One scrawled "killer" in chalk on the scientist's doorstep, while another hurled insults through a bullhorn and announced, "Your neighbour kills animals!" Someone shattered a window.

Borrowing the kind of tactics used by anti-abortion demonstrators, animal rights activists are increasingly taking their rage straight to scientists' front doors. Over the past couple of years, more and more researchers who experiment on animals have been harassed and terrorised in their own homes with weapons that include firebombs, flooding and acid.

Scientists say the vandalism and intimidation threaten not just themselves and their families but the future of medical research.

Accompanying the attacks is increasingly tough talk from activists such as Jerry Vlasak, a spokesman for the Animal Liberation Front press office. In an interview, he said he is not encouraging anyone to commit murder, but "if you had to hurt somebody or intimidate them or kill them, it would be morally justifiable."

The Washington-based Foundation for Biomedical Research said researchers were harassed or otherwise victimised more than 70 times in 2003, up from just 10 the year before.

"We consider this to be a serious problem, especially when people's lives are being disrupted," said FBI agent David Strange, who oversees a domestic counterterrorism squad at the FBI's Oakland office. "We call it terrorism because it is a violent act violating federal criminal laws."