A representative for the Teamsters didn't immediately respond to CNBC's inquiry about whether outsiders are participating in the strike.
The Teamsters website says that nearly 10,000 Amazon workers have joined the organization. That represents less than 1% of the company's workforce of 1.53 million, as of Dec. 31, 2023. The union said Thursday's campaign is the largest strike against Amazon in American history.
Amazon has long opposed unions among its workforce, but efforts to organize started materializing in 2022, when warehouse workers on New York's Staten Island voted to join a union.