Angela Lynn Adams Fulton, who grew up on a farm, said GMOs lead to all sort of health problems.
"We've basically been sentenced to death, to illness, to infertility," she said.
And Aaron Davis, a military veteran, pointed out that Monsanto was one of the producers of the defoliant Agent Orange during the Vietnam War.
"They have a history of making the world's most toxic compounds," he said.
Genetically modified plants are grown from seeds that are engineered to resist insecticide and herbicides and improve crop yields. The use of GMOs has been a growing issue of contention, with health advocates pushing for mandatory labeling of genetically modified products even though the federal government and many scientists say the technology is safe.
Tom Helscher, a spokesman for the St. Louis, Mo.-based business, said Monsanto helps improve farm productivity and food quality.
"Among the challenges facing agriculture are producing food for our growing population and reducing agriculture's footprint on the environment," Helscher said. "While we respect each individual's right to express their point of view on these topics, we believe we are making a contribution to improving agriculture by helping farmers produce more from their land while conserving natural resources such as water and energy."
Opponents disagree that GMOs provide a benefit and claim that Monsanto hurts small and organic farmers by establishing a monopoly over the world's food supply and bullying them into buying only the company's seeds. They also said that genetically modified crops and the use of Monsanto pesticides have a strong correlation to the collapse of bee colonies.
Cara Warren, who lives in Marriott-Slaterville in Weber County, said she came to the protest because of the importance of finding a way to boost the food supply without the "horrific consequences."
"I'm not against science, but we need to take a look at the big picture," she said.
The Associated Press contributed to this article.