The world's largest retailer, Wal-Mart, has been ordered to pay at least $78m (ú42m) in compensation to workers who were forced to work during breaks.
A jury in a Pennsylvania court decided that Wal-Mart broke a state law by refusing to pay staff for the extra work they did.
The class action was brought by about 187,000 staff who worked for Wal-Mart between March 1997 and May 2006.
Wal-Mart said it was planning a "very broad-ranging appeal".
The former employee who headed the case, Dolores Hummel, who worked at branch of Wal-Mart owned wholesaler Sam's Club for 10 years, said she regularly had to work during breaks and after closing time because of work demands.
She estimated she worked between eight and 12 hours unpaid each month.
In the lawsuit, she said: "One of Wal-Mart's undisclosed secrets for its profitability is its creation and implementation of a system that encourages off-the-clock work for its hourly employees."
After the judgement she said she and other staff had acted because they wanted to show "how we were treated working at Wal-Mart - working off the clock and not getting paid".
A lawyer for the plaintiffs, Michael Donovan, said he would seek an extra $62m in damages because the jury had found that Wal-Mart acted in bad faith.
The jury found that the retailer had not denied the staff their meal breaks - but only rest breaks.
In December, a California court ruled Wal-Mart must pay $172m in compensation to 116,000 employees who had been denied meal breaks.
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