Mr Gunn said of the plans which are due to come into force in April: “There’s all the other taxes they’ve been putting on us, and the prices we get for our produce and what it costs in the shop, we don’t make any money.
“Then there’s food security, farmers are going out of business.”
He said his message to government was: “Sort the pledge out.”
“You said in the manifesto you would look after the farmers, which you totally haven’t, you’ve ruined the countryside,” he said.
Tyler Carter, 18, from Peterborough, was also among the campaigners gathered in Trafalgar Square.
He held a sign reading: “Dear London sorry … I’m here to fight for my future!”
When asked what the reforms would mean for him, he said: “It means my dad will be out of a job, which means I will be out of a job.
“We have worked hard for what we’ve got and don’t deserve to have it taken off us.”