Baristas and other employees of Starbucks Korea mobilized two trucks emblazoned with words of protest on Thursday, expressing their pent-up anger against the coffee chain and calling out treatment they consider inhumane.

“Starbucks Partners are not expendable. Partners, who have raised their voices for the first time in 22 years, shall not be silenced,” read the message on the sides of both trucks, which drove past symbolic locations in Seoul for Starbucks Korea. 

These included the country’s very first branch, near Ewha Womans University, and the chain’s 1,000th location, in Cheongdam-dong, southern Seoul.

Talk of the protest, the first one ever organized by Starbucks employees, emerged last week after employees complained about a recent marketing event late last month that drew long lines of customers waiting to get their coffee in reusable cups.

Starbucks Korea CEO Song Ho-seop apologized to the employees after the event in an internal email, local media reported.

"We have put great pressure on our partners due to insufficiencies in the preparation process,” the chief executive said in the email.

The company would discuss how to improve its promotional events, ensure more flexibility in personnel management and strengthen communication within the organization, he said.

Song reportedly sat down with 20 Starbucks employees that day to listen to their opinions and discuss areas where there was room for improvement, said local media.

Starbucks Korea is said to be pushing ahead with its annual reward event, which starts Oct. 17 and will award merchandise to customers who’ve purchased a certain number of drinks.