The Irish Farmers' Association is protesting proposed reforms to the Common Agricultural Policy outside Dublin Castle where EU agriculture ministers are discussing the changes.
Hundreds are attending the protest and Gardaí have closed off a section of Dame Street to traffic.
The IFA earlier said the introduction of a minimum payment per hectare will take money from the most productive farmers and reward those who produce less.
They said the EU Agriculture Commissioner's proposal for a mandantory minimum payment was not part of an agreement reached at meeting of European Agriculture ministers last March.
National livestock chairman of the IFA Henry Byrne says Minister Simon Coveney must move to protect Irish farmers.
"We're protesting to remind Minister Coveney that he has to act to protect the active producer in Ireland," Mr Byrne said.
"Moves to move single payment away from production agriculture towards a more social welfare-based agriculture is wrong.
"It'll be wrong for jobs and exports."