LOS ANGELES -- Marches and rallies against seed giant Monsanto ( MON) were held across the U.S. and in dozens of other countries Saturday."March Against Monsanto" protesters said they wanted to call attention to the dangers posed by genetically modified food and the food giants that produce it. Marches were planned for more than 250 cities around the globe, according to organizers. Genetically modified plants are grown from seeds that are engineered to resist insects and herbicides, add nutritional benefits or otherwise improve crop yields and increase the global food supply. Most corn, soybean and cotton crops grown in the U.S. today have been genetically modified. But some say genetically modified organisms can lead to serious health conditions and harm the environment. Monsanto Co., based in St. Louis, said Saturday that it respects people's rights to express their opinion on the topic, but maintains that its seeds improve agriculture by helping farmers produce more from their land while conserving resources, like water and energy. The use of genetically modified organisms, or GMOs, has been a growing issue of contention in recent years, with health advocates pushing for mandatory labeling of genetically modified products even though the federal government and many scientists say the technology is safe. The Food and Drug Administration does not require the labeling, but organic food companies and some consumer groups have intensified their push for labels, arguing that the modified seeds are floating from field to field and contaminating traditional crops. The groups have been bolstered by a growing network of consumers who are wary of processed and modified foods. The Senate this week overwhelmingly rejected a bill that would allow states to require labeling of genetically modified foods. The Biotechnology Industry Organization, a lobbying group that represents Monsanto, DuPont & Co. ( DD) and other makers of genetically modified seeds, has said that it supports voluntary labeling for people who seek out such products. But it says that mandatory labeling would only mislead or confuse consumers into thinking the products aren't safe, even though the FDA has said there's no difference between GMO and organic, non-GMO foods.