LOS ANGELES -- Protesters rallied in dozens of cities Saturday as part of a global protest against seed giant Monsanto and the genetically modified food it produces, organizers said.

Organizers said "March Against Monsanto" protests were held in 52 countries and 436 cities, including Los Angeles where demonstrators waved signs that read "Real Food 4 Real People" and "Label GMOs, It's Our Right to Know."

Genetically modified plants are grown from seeds that are engineered to resist insecticides 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 United States today have been genetically modified. But critics say genetically modified organisms can lead to serious health conditions and harm the environment. The use of 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 'March Against Monsanto' movement began just a few months ago, when founder and organizer Tami Canal created a Facebook page on Feb. 28 calling for a rally against the company's practices.

"If I had gotten 3,000 people to join me, I would have considered that a success," she said Saturday. Instead, she said an "incredible" number of people responded to her message and turned out to rally.

"It was empowering and inspiring to see so many people, from different walks of life, put aside their differences and come together today," Canal said. The group plans to harness the success of the event to continue its anti-GMO cause.

"We will continue until Monsanto complies with consumer demand. They are poisoning our children, poisoning our planet," she said. "If we don't act, who's going to?"

Protesters in Buenos Aires and other cities in Argentina, where Monsanto's genetically modified soy and grains now command nearly 100 per cent of the market, and the company's Roundup-Ready chemicals are sprayed throughout the year on fields where cows once grazed. They carried signs saying "Monsanto-Get out of Latin America"

Monsanto Co., based in St. Louis, said 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 such as water and energy.

The Food and Drug Administration does not require genetically modified foods to carry a label, 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 U.S. 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. 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.

However, state legislatures in Vermont and Connecticut moved ahead this month with votes to make food companies declare genetically modified ingredients on their packages. And supermarket retailer Whole Foods Markets Inc. has said that all products in its North American stores that contain genetically modified ingredients will be labeled as such by 2018.

Whole Foods says there is growing demand for products that don't use GMOs, with sales of products with a "Non-GMO" verification label spiking between 15 per cent and 30 per cent.