At least 20,000 are expected to descend on the capital for a rally backed by Jeremy Clarkson and other high-profile farmers.
Organisers say children on toy tractors will be leading the procession from Whitehall to Parliament Square to highlight "the impact of the devastating Budget on the future of farming and the countryside".
Protesters have been asked to wear boots and wellies and bring produce for food banks but not to arrive on tractors and disrupt roads. One of the organisers, Olly Harrison, said: "We are not ready to go full French."
Farmer Andrew Ward, who will be at the protest, said: "Our job is to put food on people's plates and we should be planting next year's crops. But the march today is too important to miss.
"The Budget threatens the destruction of the farming industry as we know it and family farms and other businesses will be lost. If this happens, we can't feed people and wildlife will also suffer. It's that simple.
"We haven't seen a threat to our rural way of life like this in decades. We have to make sure our voices are heard and the Government listens."
Passionate
Ten minutes' walk away, the National Farmers' Union will be holding a mass lobby of MPs. It says 1,800 members - three times as many as planned - will meet backbenchers to urge action in three lots of 600 at Church House by Westminster Abbey.
NFU president Tom Bradshaw urged protesters to be "passionate" and "peaceful".
He said: "We want the very best of the countryside to come to London to make sure that nobody can be in any doubt what this policy means for you, your family, your future." He is due to address farmers at the rally, where he is expected to say he has never seen the industry "this angry, this disillusioned and this upset".
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