A state watchdog has been accused of criminalising farmers trying to defend their livelihoods.
The Irish Farmers' Association (IFA) said a raid at its head offices by the Competition Authority was an "outrageous attack" on cattle owners across the country.
IFA president John Bryan told 4,000 farmers protesting outside the Department of Enterprise, Jobs and Innovation that the authority is going after the wrong target.
Retail giants and processors were hiding multimillion-euro profits from the public and dictating the price paid to suppliers, he said, adding: "Any law that protects retailers and criminalises farmers is wrong. Our competition law is flawed and the law must be changed."
The Competition Authority, the body responsible for enforcing Irish and European competition law in Ireland, said any attempt to directly or indirectly fix prices is against the law.
It said its search of the IFA offices on May 13 was part of an investigation into price-fixing in the liquid milk market, and follows disruption by groups of farmers at several shops.
The authority added: "The work of the authority is focused on protecting competition in order to benefit consumers and the economy as a whole, and it uses its statutory powers to that end."
But Mr Bryan said suppliers are forced to pay retailers "hello money" in order to get their product on the shelf and also had to foot the bill for unsustainable discounting and two-for-one offers.
A spokeswoman for Enterprise Minister Richard Bruton has said Mr Bruton will meet the IFA to discuss a report on a code of practice for the grocery sector but insisted that holding talks about the Competition Authority raid would be inappropriate.
Fianna Fail spokesperson on agriculture Michael Moynihan called on the Government to introduce fair trade legislation which would put in place a mandatory code of practice governing the relationship between retailers and suppliers.