As the Teamsters' cross-country Christmastime strike against Amazon entered its second day on Friday, the union said that "momentum continues to mount" as workers seek "fair treatment" from the online retailer.
"The Amazon Teamsters movement grows bigger and stronger every day and will not be stopped," the International Brotherhood of Teamsters said in a statement posted on social media late Thursday.
Workers affiliated with the Teamsters began striking at Amazon facilities across the country early Thursday. The union said thousands of workers were walking off their jobs at facilities in New York City, Atlanta, Southern California, San Francisco and Illinois, but did not provide specific numbers.
Teamsters President Sean M. O’Brien is expected to join a picket line in California's City of Industry on Friday, the union said.
MORE: Amazon strike could lead to delays in delivery of holiday giftsAmazon said the strike was not expected to impact operations and claimed the strikes were being attended by outside organizers. Kelly Nantel, director of Global Corporate Issues and Media Relations at Amazon, said the company had not seen an impact on deliveries.
"Thankfully, the vast majority of our employees and the drivers who deliver on our behalf came to work today to do what they do every day," Nantel said on Thursday. "They're doing a great job of working for their customers and their communities and as a result of their hard work."
The strike, which the Teamsters referred to as the largest strike in history, arrived during the busiest shopping season of the year, less than a week before Christmas.
In addition, the Teamsters said local unions were also picketing "hundreds" of Amazon Fulfillment Centers nationwide.
Overall, nearly 9,000 Amazon workers, across 20 bargaining units, have affiliated with the powerful Teamsters union, according to the union. The striking workers represent less than 1% of the company's 1.5 million employees worldwide, including 800,000 in the United States.
The National Labor Relations Board officially certified the union representing workers, but Amazon has appealed that ruling. The union said Amazon ignored a Sunday deadline for contract negotiations to begin.
"If your package is delayed during the holidays, you can blame Amazon's insatiable greed," O'Brien said in a statement Thursday announcing the strike. "We gave Amazon a clear deadline to come to the table and do right by our members. They ignored it."
In a statement to ABC News, an Amazon spokesperson said the Teamsters illegally coerced workers to join the union. The company in a statement described the people walking the picket lines as "almost entirely outsiders—not Amazon employees or partners."
"The truth is that [the Teamsters] were unable to get enough support from our employees and partners and have brought in outsiders to come and harass and intimidate our team, which is inappropriate and dangerous," Amazon said. "We appreciate all our team’s great work to serve their customers and communities, and are continuing to focus on getting customers their holiday orders.”
Amazon's market cap is $2.35 trillion. Shares of the retailer ticked up $2.77 on Thursday, climbing about 1.26%. The stock was down about 2 in premarket trading on Friday.
ABC News' Taylor Dunn and Soo Youn contributed to this report.