They gather secretly in the night, then descend on farmers' fields at first light, trampling and scything down crops before anyone can stop them.
It has become a ritual activity in Britain this summer, carried out by militants who oppose genetically modified crops. Their protests are part of a snowballing campaign across Europe against what tabloid newspapers have termed "Frankenstein foods."
More than 40 test sites at which genetically modified crops have been grown in Britain have been destroyed this year, leaving only a handful intact.
While foods from genetically modified plants are in common use in the United States, public opinion in Europe seems overwhelmingly alarmed by warnings that they could contaminate the countryside and threaten human and animal safety. In Britain, for example, one recent poll showed 79 percent of people opposed to such foods, which are produced from plants that have been genetically altered to produce higher yields and resist pests.
The European reaction may be due in part to the recent scare over mad cow disease in Britain, which spread to humans and caused some deaths, and over the discovery of Belgian meat contaminated with cancer-causing dioxins.
The opposition to foods from genetically modified plants could sharpen trade disputes between the United States and Europe, which already have broken out over such issues as banana imports and cattle fed with hormones to speed their growth.
U.S. farmers have planted a little more than half their soybean fields with genetically modified beans. Washington has said that, within five years, virtually all U.S. agricultural exports will be genetically modified or combined with bulk commodities that have been altered. If that is the case, U.S. exporters may find the European door shut to their crops.
Already in Britain and elsewhere, supermarket chains have taken foods with genetically modified content off their shelves and put labels on other products certifying that they contain no genetically altered food.
One of the most widely publicized attacks on crops in Britain was carried out in Norfolk by the environmental group Greenpeace and its leader, Lord Melchett. Melchett and 27 followers were arrested after they stomped and hacked their way through a corn field, destroying much of the crop before police and the farmers who owned the land could intervene.
While scientists and some newspapers have denounced such destruction, saying field trials are essential to determine if genetically modified crops are safe, Melchett dismissed the trial as "a fake science project, a shoddy field of living pollution."
He was once detained by the Chinese for hanging a banner in Tiananmen Square in Beijing to commemorate the slaughter of student demonstrators there, and has headed Greenpeace since 1989.
Other test sites have been destroyed in Lincolnshire and Yorkshire. In one instance, activists went to the wrong field and uprooted crops that had not been genetically modified.
The protesters represent a variety of backgrounds and causes. They include organic farmers, students, anarchists, environmental groups and ordinary middle-class citizens who say they have not previously been involved in protest movements.
Opposition to genetically modified crops mushroomed in Britain last February after a study reported that rats fed genetically modified potatoes suffered a weakened immune system and damage to vital organs.
Prime Minister Tony Blair was quoted last February as saying he believed genetically modified foods are safe. But, as opposition has grown, the government has been more reticent.
In Rome last week, Greenpeace placed a giant, condom-covered corn cob in front of the Health Ministry to protest genetically modified foods. Greenpeace spokesman Alessandro Gianni said Italy had become one of the leading countries in Europe for testing such crops, and labeling of such products in grocery stores does not exist.
Italy does not ban genetically modified crops from being grown, but they can be planted only with Agriculture Ministry approval. Government officials said there was no cultivation of such crops at the moment because approval has not been given.
Gianni said Greenpeace would start a campaign in supermarkets in September, telling consumers they were being used as guinea pigs and already eating a number of genetically modified foods.
Switzerland, he said, has requested a 10-year moratorium on commercial-scale genetically engineered crop production. He also said some restrictions on such crops already are in place in Austria, Greece, France, Luxembourg, England and Denmark.