Rallies were planned at 50 locations in 27 states. In New York, about 100 workers marched from the main office on 8th Avenue to a Staples store about five blocks away, carrying signs and chanting, "Hey, hey, ho, ho, Staples deal has got to go."
In Washington, D.C., more than 200 people gathered at a Staples, drumming on buckets and holding signs that read: "Stop Staples. The US Mail is Not for Sale."
One of them, postal service maintenance mechanic Robert Black, called the pilot program "a back-door way of privatizing the post office" and taking away jobs from postal workers.
"It seems as though they are doing whatever they can to break down the union," he said.
Last year, Framingham, Mass.-based Staples Inc. began offering postal services under a pilot program that now includes some 80 stores. The American Postal Workers Union objects, because it says well-paid union workers have been replaced by low-wage nonunion workers. (A union spokesman said postal workers make $25 an hour on average, far more than retail clerks.) The union also worries it will lead to post office closures.
John Hegarty, president of the National Postal Mail Handlers Union, which represents about 45,000 mail handlers, also said the outsourcing endangers the sanctity and security of the mail.
"We are highly trained, skilled postal employees and they want to give it to employees who really don't know anything about the mail," he said.
Staples customer Jon Lenzner in Washington agreed that security was a concern.
"While the majority of postal workers are honest, it enlarges the pool of people who can take private, personal information," said Lenzner, a prosecutor. "You have, in essence, doubled the pool of people who can steal your mail."
Mark Dimondstein, president of the American Postal Workers Union representing 200,000 employees, called the Staples partnership "a dirty deal."
"It represents a shift of good, living-wage jobs to low-wage jobs," Dimondstein said.
Staples spokesman Mark Cautela would not address the workers' concerns, only saying the store is always testing new ways to serve its customers.
The dispute comes as the financially struggling Postal Service looks to cut costs and boost revenues.
Postmaster General Patrick Donahoe has said the Staples program has nothing to do with privatization. Rather, it's a "direct response to the changing expectations of customers who demand greater convenience and a one-stop shopping experience." It's also an opportunity "to grow the business," the Postal Service said in a statement Thursday.
Aside from Staples, the Postal Service has roughly 65,000 other retail partner locations around the country - such as CVS pharmacies and Wal-Mart stores that sell postal products. The Staples program, however, allows customers to buy stamps, send packages and use Priority and certified mail.
The aim is to increase access "to customers in locations where they already shop," the agency says.
The service lost $5 billion in the 2013 fiscal year and has been trying to get Congress to pass legislation to help with its financial woes, including an end to Saturday mail delivery and reduced payments on retiree health benefits. It lost $15.9 billion in the 2012 budget year.
Associated Press writers Stacy A. Anderson and Pauline Jelinek in Washington, and Michael Sisak in New York City contributed to this report.