Workers at hundreds of Starbucks outlets across the United States went on a one-day strike during "Red Cup Day," considered to be one of the busiest days for the global coffee chain, requesting better working conditions, according to the labor union Workers Union.

The walkout comes on the coffee chain's Red Cup Day event, during which Starbucks hands out free red-colored, reusable, holiday-themed cups to customers on their coffee purchases.

Starbucks said on Thursday its stores in the U.S. were "open," adding that "a few dozen stores with some partners (were) on strike," but more than half of those stores were open this morning, "serving customers."

About a dozen workers picketed outside Starbucks' Astor Place outlet at the New York University's campus chanting "no contract, no coffee" and other rhymes. Meanwhile, Astor Place continued to fill with NYU staff and students placing orders.

Red Cup day has typically been a major driver of store traffic, with Placer.ai data showing that visits to U.S. Starbucks stores on the day last year jumped 94% over the daily average for the full year.

Workers United, which represents more than 9,000 Starbucks employees at about 360 U.S. stores, has said the event was one of the "most infamously hard, understaffed days," as drink orders pile up and employees end up on the receiving end of abuse from frustrated customers over long wait times.

Mary Boca, 22, Astor Place, New York, a barista, said she wants to see higher pay and more staff at Starbucks.

"I have heard our managers saying they need to hire 12 people. At a peak period, that's a lot of people to be out of."

Boca said that her Starbucks location does not allow customers to tip, which leaves her without an extra $100 in each paycheck.

Edwin Palma Solis, 24, a worker at Astor Place, said he thinks the inability of customers to tip at the store has deterred some potential hires from joining the location.

Starbucks has nearly 10,000 U.S. company-owned locations, and according to the company, less than 3% of those stores are represented by a union.

Last year, workers at more than 100 U.S. company-owned Starbucks locations held a one-day strike on Red Cup day.

Earlier this month, Starbucks said it would raise hourly pay for its U.S. retail workers by at least 3% from 2024, which employees criticized, calling it "tone deaf" given Starbucks' 11% increase in fourth quarter revenue and the recent wage hikes won by autoworkers.