Berlin — Workers at Amazon distribution centers in Germany have gone on strike for better pay on Black Friday, one of the busiest days of the year for the online retailer.

Union ver.di said Friday that the walkouts began overnight at six distribution centers across the country, with some due to last until Tuesday. The group tweeted that workers are "striking for better wages, safer jobs and a collective agreement" at Amazon.

Union representative Orhan Akman said Amazon staff want to send a signal that their work can't be bought with the kind of extreme discounts Amazon offers around this time of the year.

Akman also accused the company of putting "extreme pressure" on staff that's causing some to fall ill.

Amazon said in an emailed statement that "packages are arriving punctually, because the bulk of employees is dealing completely normally with customers' orders today."

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It said that it offers pay "at the upper end of comparable jobs" and other benefits.