Tens of thousands of people marched in cities across the world on Saturday to protest against the American biotechnology giant Monsanto and its genetically modified crops and pesticides.

The third annual March Against Monsanto – begun by the Occupy movement – was held in around 400 cities in more than 40 countries from the Americas to Africa and Europe.

About 2,500 people staged anti-Monsanto protests in the Swiss cities of Basel and Morges, where the company has its headquarters for Europe, Africa and the Middle East.

Up to 3,000 protesters, rallied by environmental organisations including Greenpeace and anti-capitalist group Stop TAFTA, gathered in Paris, with Monsanto’s market-leading herbicide Roundup the main targets of protesters’ anger.

The controversial product’s main ingredient was recently classified as “probably carcinogenic to humans” by the World Health Organisation.

“Looking for mass suicide? Go for Roundup,” read one placard at another French protest in the western city of Rennes.

In Burkina Faso, around 500 people marched in the capital Ouagadougou against the US giant, which introduced GM cotton into the west African country in 2003.

Demonstrators demanded a 10-year moratorium on the planting of Monsanto seeds so “independent research can be conducted” into the effects of the technology.

Up to 1,000 anti-Monsanto activists also gathered in front of the European parliament in Strasbourg as the sun was setting for a minute’s silence “in homage to the existing and future victims poisoned by pesticides,” according to the organisers.

There were similar scenes in Los Angeles and Rio de Janiero.

Up to 500 protesters, including families with small children, took part in a colourful rally under the sun in Los Angeles.

“I’m not a science experiment,” read the sign of a young girl in a pushchair, while demonstrators chanted: “Hell no, GMO!”

“Monsanto is doing terrible things and that’s why we’re here,” said protester Carole Walker.

Megan Cliburn added: “We should be able to know what’s in our food when we are eating; what we are putting in our body.”

About 250 people danced and sang in a noisy demonstration in Rio, accusing Monsanto of “bioterrorism”.

In Chile’s capital Santiago around 1,000 people demanded the withdrawal of Monsanto from the country and the end of production of genetically modified foods.

“We do not want GMOs on our plates,” said Ivan Santandreu, president of the movement Chile sin Transgenicos.

Monsanto did not immediately reply to a request for comment.