Postal workers protested Thursday morning at Staples stores in South Carolina and 26 other states to blast a deal that establishes mini post offices within the office-supply giant's retail outlets.

The company introduced a pilot project in November that set up mail counters, staffed by its employees, in 82 of its stores. Staples has plans to expand the service to 1,500 locations, according to a statement from the American Postal Workers Union.

The Postal Service, which is not publicly funded, has struggled for years to stay profitable against the rise of email and online document sharing, as well as competition for package deliveries from large companies like UPS and FedEx.

Darleen Reid-DeMeo, a Postal Service spokeswoman, said partnerships with retailers could help stabilize the business.

"Staples joins more than 65,000 retail partner locations around the country that currently offer a variety of postal products and services, in order to increase access and convenience to customers in locations where they already shop," Reid-DeMeo said. "This retail partnership program could be an innovative step towards generating revenue to ensure the long-term viability of the Postal Service."

The postal union fears that "retail partnerships" could lead to more office closures. It also said that putting mail stations in stores without staffing them with trained postal workers threatens the reliability of the mail system.

"The American people have a right to know that their mail is handled by highly trained uniformed postal employees who have taken an oath to protect the sanctity of the mail and who are accountable to the people of the country - whether it's at the post office or an office-supply store," Mark Dimondstein, union president, said in a statement Thursday.

Demonstrations were held at about 50 U.S. Staples locations.

About 12 postal workers, AFL-CIO representatives and other labor rights advocates gathered Thursday morning at the retailer's store at 4464 Devine St. in Columbia.

Other regional protests took place outside a handful of stores in Florida, and one in Atlanta.

In Washington, D.C., more than 200 people gathered, drumming on buckets and holding signs that read: "Stop Staples. The US Mail is Not for Sale."

Gwendolyn Allen, who helped organize the South Carolina protest, has been an employee of the West Columbia post office for 25 years.

"The message to the American people is, you're going to spend the same amount of money to process your mail at Staples, without Postal Service employees and without the same level of security that you would have at a post office," she said.

Staples declined to comment on the deal with the Postal Service.

"As a matter of policy, we don't provide details on our pilot programs or on our agreements with vendors," said spokesman Mark Cautela.

Reach Abigail Darlington at 937-5906 and follow her on Twitter @A_Big_Gail