The union Unite said members of the public had joined the demonstration in Birmingham in an attempt to stop Kraft changing the 200-year-old company "beyond recognition" after the takeover is completed next week.
Unite said Kraft must give assurances that the interests of the workforce will not be swept aside and that the quality of products will be safeguarded.
Unite national officer Jennie Formby said: "Our members and their friends across the Midlands deserve a voice in this takeover. They are angry that to date they have been forced to look on from the sidelines as the big money players carve up their company."
Prime Minister Gordon Brown has said he is "determined" to secure British jobs at the company.