Shopping with Amazon on Prime Day, likely to be one of the online retailer's biggest sales days of the year, may mean crossing a digital picket line.

Amazon workers in Minnesota and Germany are striking in protest of working conditions and wage practices, just as Prime Day kicks off.

Those in Europe have staged protests during sale days in past years. But the action in Minnesota is the first major strike of workers in the United States during the company's annual Prime Day event. It may also be a sign that the company's increase to a $15 minimum wage last year is not enough to satisfy workers' needs.

This year's Prime Day could bring in as much as $5.8 billion in sales globally, according to Coresight Research. Last year, Amazon reported that consumers worldwide purchased more than 100 million products. This is also the first time Amazon is holding the midsummer sale since it promised in June to provide one-day shipping on select items to Prime members.

"Amazon has the means to do so much more for all of its employees, and we would like to see less physically and mentally draining jobs," William Stolz, who has worked in an Amazon warehouse for over two years, said in an interview from Minnesota with CNBC's "Closing Bell."

"We would like to see workers have better job security, better treatment for those that are injured, bringing back some of the benefits and bonuses that we used to have," he said.

The fulfillment center workers in Shakopee, Minnesota, began walking out at 3 p.m. ET Monday and planned to strike for a six-hour period that overlapped with the morning and evening shifts. There were about 75 people outside the facility at 5 p.m. ET Monday, chanting "Amazon, hear our voice!" and "We work, we sweat, Amazon workers need a rest!" Some Seattle tech employees also attended the strike in solidarity.

Amazon said 15 employees participated in the protest.