More than 1,000 people rallied at midday Monday in front of Oakland's federal building to make speeches about what they felt was a lack of regulation of genetically modified foods by the Food and Drug Administration.
Inside the building, leading food experts discussed the issue with the FDA.
"Genetic contamination is forever," said organic farmer Laura Trent, whose sign read, "Get your pig gene out of my tomato." "No scientist has proven it safe, and most people don't even know it's happening."
Genetically modified food is found supermarkets and in many restaurants. An estimated 57 percent of the soybeans and 30 percent of the corn planted in the United States this year was genetically engineered to resist pests or herbicides.
State Sen. Tom Hayden said the FDA has dragged its feet in regulating such foods.
"Now we're playing catch-up, just now seeing the long-term effects of these foods," he said.
Some demonstrators both inside and outside called for a complete ban on genetically modified foods. But labeling appeared the most likely response.
The experts addressing the FDA panel generally supported labeling, but differed over whether labels should be mandatory, and over the definition of a "modified" food.
Rhona Applebaum, of the National Food Processors Association, thought more vigorous scientific review and voluntary labeling would help consumers feel better about the foods.