Members of the Boycott Workfare campaign protested outside the Welfare to Work conference last week dressed as a chain gang and brandishing signs accusing supermarkets of paying claimants on work placements only 3p a minute to stack shelves.
Sainsbury's said it had employed thousands of people following the work placements.
Tesco said it was providing 3,000 four-week-long work placements this year.
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