Industry supporters launched a spirited discussion about golden rice, named for its yellow hue and because it is genetically engineered to produce Vitamin A in the hope that developing nations can use it to stave off malnutrition.
"We could not have come up with a better example of what biotechnology is all about," said Mike Phillips, a spokesman for the Biotechnology Industry Organization. "It's a wonderful story of the public and private sectors have come together."
Critics call it "Frankenfood." They view golden rice and other genetically modified foods as potential health hazards, and argue that not enough research has been done to determine whether they are really safe.
"The biotech industry is conducting a real-time experiment with our biosphere," said Shannon Service, 26, of Boulder, Colo., who was dressed as a monarch butterfly. "They don't know the results, they can't possibly know the results."
Service was among hundreds of demonstrators who marched through the city Sunday, some dressed as ears of corn or genetically engineered tomatoes, and carrying signs with slogans like "Biocide is Homicide."
The protest was much smaller than organizers expected, and largely peaceful.
Police have kept their distance from protesters but maintained a strong presence. They said they were determined to avoid a repeat of the 1999 World Trade Organization protests in Seattle, which led to more than 600 arrests and caused $2.5 million in downtown property damage.
The protesters' views are countered by groups such as the Grocery Manufacturers of America, which says genetically altered crops reduce the amount of water and pesticides needed to grow the nation's food.
"The same people who are marching against biotechnology are the same people who marched against pesticides several years ago," said Gene Grabowski, spokesman for the Grocery Manufacturers of America.
Other hot-button topics on the agenda for the 15,000 participants at the three-day BIO 2000 conference included embryonic stem cell research and the use of animal organs and cells to treat human illnesses.
Protest organizers had expected several thousand demonstrators and blamed police and the media for the smaller turnout.
"The whole downtown of San Diego has been militarized," said Han Shan, spokesman for the Ruckus Society, a group that trains protesters in nonviolent demonstrations. "There are a lot of people out here who feel we're being criminalized for simply expressing concern with biotechnology."
Police said their main concern was the anarchist groups that have disrupted previous anti-globalization protests. The groups typically stand out because members dress in black and wrap their faces in ski masks or bandannas.
Several such people marched with demostrators Sunday, but police spokesman Dave Cohen said there were no major confrontations with police.
Police arrested eight people during the weekend, including two men taken into custody Sunday for investigation of carrying concealed daggers, Cohen said. Two others were arrested for allegedly vandalizing a police car, and others were stopped for carrying protest signs outside the demonstration area.
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