More than 300 farmers dumped around 100 tonnes of manure and rotten vegetables in the centre of Chartres in protest at falling food prices.
Using some 30 tractors, the farmers dumped the slurry and rotting food in front of the offices of the agriculture department and town hall.
The protesters, from the FDSEA union, were "fed up" with the "increasingly crazy constraints and charges" faced by farmers, said union leader Jean-Michel Gouache.
He denounced the "collapsing prices of cereals, milk and vegetables, caused in part by the sanctions on Russia" as well as rising fertiliser prices.
Russia imposed a ban on agricultural products from the EU earlier this year in response to European sanctions over the Ukraine crisis.
The protest in Chartres, southwest of Paris, comes ahead of widespread farmer strikes due across the country on Wednesday.