The walkout began last Friday in Seattle, Los Angeles, and Chicago. On Monday, media reported that the walkout spread across 10 states.
The company's workers reportedly boosted their efforts all across the US on Tuesday, which led to the closure of at least 170 stores.
At the same time, Starbucks Workers United said early on Tuesday that it expected over 5,000 people to walk out, leading to closures of 300 stores in 45 states.
Starbucks employees are calling for increased salaries, expanded staffing, and more flexible scheduling, the union states. CNN has reported that the strike may expand to all of the United States if the management and the union fail to come to an agreement.