Up to 1,000 demonstrators, some dressed as ears of corn or genetically engineered tomatoes, staged a colorful but largely peaceful protest Sunday on the opening day of a biotechnology trade show. Many demonstrators said they are concerned that businesses are introducing genetically modified crops and seeds into the food supply without knowing the long-term consequences.

``The biotech industry is conducting a real-time experiment with our biosphere,'' said 26-year-old Shannon Service of Boulder, Colo., who was dressed as a monarch butterfly. ``They don't know the results, they can't possibly know the results.

The monarch butterfly represents that well.'' Research has suggested pollen from genetically engineered corn can be toxic to the butterflies, whose favorite food, milkweed, grows in and around corn fields. The altered corn produces its own pesticide to kill an insect pest. Earlier this year, a panel of scientists who advise the federal government urged more research into such crops to determine their effect on the environment.

The estimated 750 to 1,000 demonstrators gathered in Balboa Park for an afternoon march to the San Diego Convention Center, where participants registered for the Biotechnology Industry Organization's annual conference.

They listened to speeches, performed street theater and hung banners. One man was dressed as a tomato and wore a sign reading ``I was a test tube veggie.'' Another carried a sign reading ``Biocide is Homicide.

'The protests were largely peaceful. Police arrested two people for carrying concealed knives, police spokesman Dave Cohen said. Outside the convention center, a black-dressed protester burned an American flag. Some spectators disagreed with protesters' concerns about genetically altered crops. ``I have no problem with their right to protest, but they have no clue what they're talking about,'' said Jessica Van Wert, 52, of San Diego.

``People are starving. We so desperately need technology to step up and feed the world.'' Protest organizers had expected several thousand participants and blamed the lower turnout on police and the media. They ``drummed up a tremendous amount of paranoia and hysteria,'' said Han Shan, spokesman for the Ruckus Society, which trained protesters in nonviolent tactics for the event.

``The whole downtown of San Diego has been militarized,'' Shan said. ``There are a lot of people out here who feel we're being criminalized for simply expressing concern with biotechnology.''

Police said they were determined to avoid a repeat of the 1999 World Trade Organization protests in Seattle, which caused $2.5 million in downtown property damage. More than 600 people were arrested.

Uniformed officers kept their distance from protesters in the park, but dozens of police cars, motorcycles and officers - some in riot gear - were available on nearby streets. Police followed demonstrators on their march to the convention center.

Police said their main concern was the anarchist groups that have disrupted previous anti-globalization demonstrations around the country. Members of those groups often dress in black and wrap their faces in ski masks or bandannas, or carry gas masks. Several were among the marchers Sunday. Conference organizers acknowledge the protesters' concerns but defended the industry and the trade show, which is expected to draw 15,000 participants.

Michael J. Phillips, the organization's executive director of food and agriculture, said that research showing the crops are safe is ``sound and irrefutable.'' Eric Anderson, an ornamental seed farmer from north San Diego County, came to Balboa Park to counter the protesters' message.

``We let the protests define our work for far too long,'' he said, saying biotechnology makes his work easier and safer. ``We're able to do more with less - less chemicals and more environmentally friendly.''