Protests against seed giant Monsanto have been held across the US and in dozens of other countries.
"March Against Monsanto" organisers said they were calling attention to the dangers posed by genetically modified food and the companies that produce it.
Protests were being held in more than 250 cities on Saturday.
Genetically modified plants are grown from seeds engineered to resist insecticides and herbicides, add nutritional benefits and improve crop yields.
The company also produces many highly toxic chemicals including pesticides, plastics and artificial food additives.
Some believe they can lead to health problems and harm the environment.
Opponents have pushed for mandatory labelling, although the US government and many scientists have said that the technology was safe.
The Monsanto company, based in Saint Louis, said on Saturday that it respected people's rights to express their opinion, but believed its seeds helped farmers produce more food, while conserving water and energy.