The farmers, some on tractors, surrounded sites run by major processors including Arla's centres in Ashby-de-la-Zouch in Leicestershire and Leeds, as well as the Robert Wiseman Dairy in Bridgwater, Somerset.
"It just shows the strength of feeling there is."
Describing Leeds-based dairy firm Arla as the target of the protests, he added: "They are one of the big three who are trying to kill the dairy industry altogether."
The protesters were at the sites until late last night and vowed not to move until senior managers talked to the farmers.
Their cause has been receiving backing from celebrity chefs Jamie Oliver and Hugh Fearnley-Whittingstall, who called on shoppers to boycott supermarkets that pay an unfair price to farmers.
Speaking outside the Arla plant in Leeds, farmer Stephen Britten told BBC Look North: "If things don't change there'll be lot of farmers go out of business in the next 12 months.
"It shouldn't happen. There's the young generation. I was given a chance to milk cows so why can't they?"
Farming Minister Jim Paice said: "These price cuts are a severe blow for dairy farmers.
"Government cannot and should not set prices but I will do everything in my power to get all levels of the supply chain to make the real changes needed to guarantee the industry's long-term future.
"Farmers and processors need to work together through an industry code of practice on contracts, and retailers have to help shift the focus away from short-term practices which are completely unsustainable."
