Amazon.com workers at seven U.S. facilities walked off the job early on Thursday during the holiday shopping rush, aiming to pressure the retailer into contract talks with their union.

Amazon warehouse workers in San Francisco and six other facilities around the nation went out on strike Thursday to try to force the e-commerce giant to negotiate with them. As part of a nationwide labor action organized by the Teamsters union — which the Teamsters called the largest-ever against Amazon — about 40 people picketed outside of the company’s facility at 749 Toland St.

Coordinated strikes ahead of Christmas in a handful of cities including San Francisco and New York see the Teamsters challenge the nation’s second-largest private employer.