Tractors trundled alongside as children on toy trucks representing Britain's farming future joined the huge rally.
Protesters waved placards reading "Starmer the farmer harmer" and "No farmers, no food, no future" just yards from Downing Street, which insists the changes are "balanced and proportionate".
Some farmers warned they would be forced to sell land to pay the "tractor tax" and are threatening to strike over the pressures of the policy, which applies from 2026.
Farms currently qualify for 100% relief on inheritance tax on agricultural and business property but, under Labour's plan, it will be imposed on farms worth more than £1million. Assets above the threshold will attract a 20% tax rate, rather than the normal 40%.
Kathryn Shorrock, 56, who farms sheep and cattle on 200 acres near Melton Mowbray, Leics, said: "We risk losing a fifthgeneration family farm.
"We do not want to be the ones to shut the farm gates for the last time.
"That would be a horrible thing to have to do just because the Government can't see the importance of food security."
Her daughter Isabella Shorrock, 21, who stands to inherit the farm, told of her despair over her future.
She said: "I'm anxious and disappointed. I've had quite big aspirations since childhood to take responsibility for the farm.
"The next generation of farmers should be able to do that but it's turning to fear and dread. It's heartbreaking."
She said the UK has some of the best food and animal welfare standards in the world but these could be at risk as the proposed tax change bankrupts farms and leaves Britain more reliant on food imports.
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