Eight protesters were arrested as more than 1,500 people marched in the streets of the state capital at the start of the three-day event.
Secretary of Agriculture Ann Veneman told agriculture ministers, scientists and health care experts from 120 countries that biotechnology can help developing nations reduce hunger while improving nutrition and their economies.
"Biotechnology is already helping both small and large-scale farmers around the world by boosting yields, lowering costs, reducing pesticide use and making crops more resistant to disease, pests and drought," Veneman said.
Demonstrators claim biotechnology isn't the antidote to complex global food problems and say the conference is a means for the United States to lower trade barriers and expand the use of genetically altered crops.
They rallied on the steps of the state Capitol under the scrutiny of hundreds of police and California Highway Patrol officers and then spread out through downtown. Authorities said eight protesters were arrested but had no information on possible charges.
Demonstrators included chefs in aprons and white hats banging utensils on saucepans, as well as activists dressed as giant ears of corn, butterflies and tomatoes. Protesters carried large puppets, signs such as "Attack of the Killer Tomatoes," "Feed the needy, not the greedy," and trumpeted urban food programs, veganism and organic farming.
After the planned march, about 20 protesters doffed their clothes, danced on the Capitol steps and began an unauthorized march through downtown. The naked protesters dispersed when the highway patrol brought in buses and threatened to arrest them.
The conference, sponsored by the Agriculture Department, is focusing on farming methods, irrigation and pest management to help developing countries cut world hunger by 2015, a goal set by agriculture secretaries at the World Food Summit last year. More than 800 million people face chronic hunger or malnutrition.
The debate over genetically modified foods is intensifying, with the United States demanding that the World Trade Organization force the European Union to end its ban on genetically modified food. EU ministers did not attend the conference.
The Agriculture Department has closed the conference to the public and certain events to the media, citing security reasons.
By Kim Baca