Two leading academics at Oxford University have decided to face down threats of violence from animal rights extremists and speak publicly in favour of the building of a controversial £18m research laboratory in the city.

Although scientists are advised to remain silent for fear of attacks, Professor Tipu Aziz, a consultant neurosurgeon, and Professor John Stein, a neurophysiologist have told the Guardian they believe it is time to stand up to the radicals who have attempted to stop the project.

"I think that it is important to speak out," said Prof Aziz, whose research into Parkinson's disease involves the use of primates.

"The ALF [Animal Liberation Front] are actively now saying that anyone in Oxford is a target. They have had it all their own way for a long time. What we are seeing in Britain today is a minority dictating how the majority of this country live and that is as undemocratic a process as can be imagined. Animal research is absolutely essential to medical progress and a lot of research being done in Oxford is critical."

He and Prof Stein will address a march on Saturday, the first demonstration supporting the construction of the laboratory.

Extra police are being drafted into Oxford for the march because it clashes with another being run by Speak, the protest group opposed to building the new animal research facility.

Prof Stein, who runs the laboratory where research into Parkinson's and dyslexia is carried out, said he knew there were dangers involved in speaking at the march.

"You have to be really passionate about this to put your head above the parapet and not many do," he said. "Some of these people are loonies and do the most awful things. Let's be clear, we are all taking a risk, but I feel it is so important I am prepared to take that risk.

"The anti-vivs have had it all their own way. They have intimidated people, but the time has come to speak up and risk it. Who knows what that risk is?

"I feel passionately that animal experiments have benefited mankind enormously and almost all of the medical advances of the last 100 years have happened through animal experiments. People just don't seem to know this, it hasn't been got across."

The movement in defence of animal research at Oxford is growing amid increasing tension. In recent weeks members of the ALF have been encouraging and directing a violent campaign against university funders, students and researchers through postings on a website based in Florida. Set up by undergraduates in a rearguard action to this increasingly voluble campaign, the pro-animal testing movement is also using the internet to spread its message.

Work was restarted on the lab in November after a year's delay when the original contractor, Montpelier, pulled out amid threats and intimidation from animal activists.

Today the identity of the new contractor, which operates on South Parks Road behind a five metre (15ft) barrier remains a secret. Builders wear balaclavas and the vehicles involved are all unmarked.

Under the terms of an injunction obtained by Oxford, noisy demonstrations against the lab are allowed to take place each Thursday, within a cordon opposite the building site.

But behind the public face of the anti-lab protest anonymous extremists from the ALF are encouraging the use of increasingly violent tactics. On a direct action website, Bite Back, registered to an address in West Palm Beach, Florida, the ALF posts notices announcing attacks on anyone linked to the university and calling on supporters to "do whatever it takes" to "blow these fucking monsters off the planet".

Extra security has been offered to many researchers and leading figures at the university and students have been warned to be vigilant.

One of the tests for new electrical therapies carried out in Prof Stein's laboratory involves electrodes being placed in the brains of monkeys which have been given Parkinson's; something he says is painless because there are no pain receptors in the brain. On the Speak website this experiment is highlighted and he is accused of "inflicting the most horrific suffering on innocent creatures".

Other researchers will remain silent on Saturday, privately believing Prof Aziz and Prof Stein are taking a huge risk. One, who would not be named, said it was not even sensible to discuss animal testing anywhere publicly in Oxford for fear of being overheard by anti-vivisectionists.

Officers from Nectu, the specialist unit monitoring animal rights extremists, will watch events on Saturday along with Thames Valley police. "The track record of animal rights extremists shows there is a high level of criminality associated with it. All threats are being taken seriously," said a police source. But the police appear powerless to act against the Florida website.

The FBI says it is "aware" of the man behind it, Nicolas Atwood, a Florida-based activist but they cannot close it down because that would breach the First Amendment, protecting freedom of speech. Mr Atwood told the Guardian he was a volunteer editor for the site. "The ALF is made up of compassionate individuals who act on their conscience, not on the orders of some mysterious leader," he said.

"Bite Back's mission is to support animal rights prisoners of conscience and report on current events in the struggle. Its editors, designers and contributors, although maybe sympathetic are not responsible for any unlawful act taken in pursuit of such benevolent goals."