Outside BIO 2005, the year's biggest biotechnology conference, protesters made their feelings clear with chants and banners about animal "murderers" and "torturers".

Inside the Philadelphia Convention Centre, the world's biotech and pharmaceutical companies resolved to hit back against increasing harassment and violence by animals rights and environmental activists.

"We thought that if we just kept our heads down, the problem would go away - but that was wishful thinking," says John Gallagher, communications director for California-based Chiron, a target for activists since August 2003 when two bombs exploded outside its headquarters near San Francisco.

A growing number of corporate victims of violent protests are fighting back - through the courts and by working with law enforcement agencies. They are also defending, through public education and media campaigns, their need to use animals in research.

The Federal Bureau of Investigation (FBI) galvanised last year's BIO meeting by warning that animal rights and environmental extremists had become the leading domestic terror threat in the US. This year John Lewis, the FBI's head of counter- terrorism, underlined that message, spending two days at the conference talking to biotech and pharmaceutical executives.

Anton Setchell, the UK's National Co-ordinator for Domestic Extremism, was there too, underlining the need for international action.

"The most pernicious forms of animal rights extremism were exported from the UK," he says. "We know there are close links between extremist groups in the UK and US - and increasingly in continental Europe too - so it is imperative that we cement the working relationship between law enforcement agencies."

Timothy Morris, head of animal policy for GlaxoSmith-Kline, the UK-based pharmaceutical group, says animal rights extremism is "a global issue". Vandalism and harassment are widespread in Europe, he adds.

Arson by the Animal Liberation Front recently closed down a monkey-breeding facility in Spain, and fear of activist attacks forced the University of Maastricht in the Netherlands to drop plans to build a primate lab.

But terrorist activity is not the only threat. Frankie Trull, president of the National Association for Biomedical Research, believes "less obvious and more insidious strategies" pose a significant long-term threat to research.

"Regulatory over-burden, prolonged litigation, the poisoning of public opinion and the exodus of young scientists for safer, less controversial fields are among the secondary impacts," she says.

Ms Trull, an outspoken advocate of animal research in the US, wants the industry to take the fight to the enemy. When news broke this week that two employees of the largest animal rights group, Peta (People for the Ethical Treatment of Animals), had been charged with animal cruelty in North Carolina, she launched an attack on its credibility.

The two employees were alleged to have picked up cats and dogs from pet shelters, then killed them and dumped their bodies in the rubbish.

Eric Dezenhall, a professional crisis management consultant, is another advocate of attack as the best form of defence. "Forget everything the PR industry tells you about being nice to your attackers and engaging in dialogue," he says. "We are seeing now for the first time that you can fight back and win."

Both Mr Dezenhall and Ms Trull cite Britain's Huntingdon Life Sciences - the number one target for extremists - as "a stellar example of a company that refused to roll over and die".

Andrew Gay, HLS marketing director, says its business grew by 15 per cent last year in spite of the activists' attempts to intimidate its suppliers.

The FBI is investigating 150 cases of environmental and animal rights terrorism. "Over the past year there has been an increase in the use of incendiary devices as well as explosive devices," Mr Lewis says. "We are very lucky not to have lost a life yet."

Law enforcement agencies are optimistic about the prospects of arresting and charging significant numbers of extremists over the next few months.

Mr Setchell, who is assistant chief constable of Thames Valley Police, warns that a new coalition of activists in Britain, the Gateway to Hell campaign, is now targeting airports, seaports and international freight carriers that bring laboratory animals in and out of the country. Several international airlines have already withdrawn from the business.He says: "We want people to be alert - not alarmed."