Starbucks workers in the United States went on strike at multiple unionized locations Thursday, a move planned to affect over 100 stores in protest against the coffee giant's approach in negotiating union contracts.

Dubbed the "Red Cup Rebellion," the one-day strike led by Starbucks Workers United coincides with a popular event in which stores hand out reusable cups with certain purchases.

Instead of the branded cups, staff gave out red union cups to interested customers.

Employees could be seen protesting in the cold near Times Square in New York, chanting and holding signs of Starbucks Workers United, which represents nearly 7,000 employees across the United States.

"It's something that is worth fighting for," one of the staff, 18-year-old Jay Tosa, told AFP.

The action takes place in "response to Starbucks' union-busting tactics and refusal to bargain," Starbucks Workers United said in a statement.

The group added that workers wanted the right to organize a union free of intimidation.

"Unless Starbucks comes to the table and bargains in good faith for a fair contract, we can count on this to happen again," the union said, raising concerns over salaries and a lack of consistent schedules.

'A lot of support'

"During the morning rush hour, we got a lot of support from our regulars," said another New York-based employee Hal Battjes.

Some commuters chose to give up their morning coffee, added the 21-year-old, who had been on the sidewalk with a megaphone since 5:30 am.

A Starbucks spokesperson said the company is "aware that union demonstrations are scheduled at a small number of our US company-owned stores."

"We respect their right to engage in lawful protest activity," the spokesperson said, adding that Starbucks continues to urge the union to meet representatives at the bargaining table.