"We are not going to stop. We are not going to stop". It could have easily been said more than 20 times during a protest at the weekend against the £18m animal testing laboratory at Oxford University.

Each time the statement was met with resounding horns and cheers as the rally wound its way through the streets of Oxford to the site of the building project.

The scheme hit the headlines last week when the construction group Montpellier pulled out of the project after its shareholders received threatening letters.

The move led to calls in parliament for action against extreme animal rights activists. City investors were said to be planning to offer a £25m reward for the conviction of extremists.

The 300-strong group of protesters that assembled in Oxford's Broad Street on Saturday, however, could not have looked less like sympathisers with a campaign of terrorism. They were mostly older men and women, many wearing rabbit ears or carrying stuffed animal toys.

Near the front, by the leading banners and supported by a walking stick, was Joan Court, 85, who last week completed a two-day hunger strike over plans for the lab.

It was called a peaceful protest and caused no trouble for the 140 Thames Valley police on duty.

Mel Broughton, spokesman for the animal pressure group Speak, said the body had fought a totally legal battle against the laboratory.

On the accusations of terrorism, he said: "We know why this is - because we are winning the war. Right now we're entering choppy waters. But the waters of victory are just across the other side."

He added: "We are not going to stop. The animals will be liberated by us, and by us alone."

Robert Cogswell, another spokesman, said: "It's going to be very, very difficult to find a building contractor to replace Montpellier and, when they do, we will certainly highlight that company and we will be legally targeting them."

The crowd marched down Broad Street towards the site of the lab, leaving the double-decker Oxford tour buses unable to move, their guides halting their descriptions of the Bodleian Library to instead explain to curious tourists the issue behind the furious waving of banners.

Oxford University says the lab is a state-of-the-art facility, in which 98% of the animals used will be rodents.

Scientists there will work to develop treatments for heart disease, cancer and diabetes. Fish, primates, ferrets and amphibians may also be used in their experimentation.

Ms Court said she felt compelled to embark on her hunger strike after looking at a picture of a primate hunched over in a small cage.

"It really hit home," she said of the photograph she carried throughout Saturday's protest.

"It was archaic, harmful, and utter, sheer wickedness."

Ms Court slept in a van outside the building site while on her hunger strike, and was rewarded with a vegan stew by a supporter once she reached the end of the vigil. "By the end of the first day it was a real struggle," she said. But "by the third day I could have gone on forever".

The tenacity of the 85-year-old has been matched by fellow anti-vivisectionists who plan to protest in the city every Thursday from 1pm in an effort to ensure that the project never goes ahead.

Steve Chandler, a long-time campaigner, is optimistic that the university's laboratory will never be completed, given the tide of ill feeling against it.

"The real terrorists here are the vivisectioners and those that are mutilating animals," he said. "They are the terrorists - not us."