Post office clerks rallied outside a Staples office supply store in Center City on Tuesday.

They were complaining that the U.S. Postal Service's plans to install in-store postal counters where lower-wage Staples employees would sell stamps and accept packages would hurt postal jobs and jeopardize the safety of the mail.

The post office "is not for sale," protesters chanted at the rally organized by their union, the American Postal Workers Union.

Calling for a boycott of school supplies at Staples, the workers say the Staples initiative is part of a postal management plan to privatize the Post Office.

"We're not trying to privatize the post office," said Raymond V. Daiutolo Sr., a USPS spokesman.

He said that the Staples initiative was part of a plan to broaden access to the most common, uncomplicated mail services, particularly during nights and weekends when post offices are closed.

A year ago, the Postal Service set up counters in 82 Staples stores in five areas - the closest to here being in Pittsburgh. In August, Staples was designated an "approved shipper," allowing the program to expand in more stores.

Tuesday's rally precedes a marketing blitz by Staples, which plans events in Philadelphia on Wednesday and Thursday to sell products such as coffee supplies and snacks for employees' break rooms. No one from Staples was available to comment on the postal workers' rally.

In March, Staples said it would close 225 stores by the middle of 2015. Daiutolo said none of those stores had postal counters.

At the rally, Cynthia Heyward, 47, a postal clerk at the Nicetown post office, said the public does not realize postal clerks have at least 80 hours of training to make sure packages are handled correctly.

She said she was not surprised that customers might want to turn elsewhere after becoming frustrated by long lines at the post office.

"They are not replacing [postal workers]. We don't have enough employees to do the job," she said, accusing management of deliberately understaffing to drive customers away.

Management's aim is to shift work to low-wage retail workers and away from her colleagues, who earn a middle-class wage, said Heyward, of Somerdale, in Camden County.

Daiutolo said keeping costs low is important, because the postal service is in "dire financial straits."

Postal service unions argue that, operationally, the postal service is on stable footing financially.

Daiutolo said foot traffic is down at post offices as customers seek one-stop shopping options or buy stamps and print out shipping labels from the Postal Service's website.

"More than 42 percent of the Postal Service's total retail revenue is generated from alternative access channels," he wrote in an e-mail.