Starbucks employees at more than 100 American stores are on strike Thursday in their largest labor action since a campaign to unionize the company’s stores began late last year.

The walkouts coincide with Starbucks’ annual Red Cup Day, when the company gives free reusable cups to customers who order a holiday drink. Employees say it’s often one of the busiest days of the year. Starbucks declined to say how many red cups it plans to distribute.

The employees say they’re seeking better pay, more consistent schedules and higher staffing levels in busy stores. Stores in 25 states planned to take part in the labor action, according to Starbucks Workers United, the group organizing the effort. Strikers are handing out their own red cups with union logos.

Starbucks, which opposes the unionization effort, said it is aware of the walkouts and respects its employees’ right to lawfully protest. The Seattle company noted that the protests are happening at a small number of its 9,000 company-run American locations.

“We remain committed to all partners and will continue to work together, side-by-side, to make Starbucks a company that works for everyone,” the company said Thursday in a statement.

Some employees planned to picket all day while others will do shorter walkouts. The union said the goal is to shut stores down during the strikes, and noted that the company usually has difficulty staffing during Red Cup Day because it’s so busy.

A shift manager at a Starbucks store at Brighton, Massachusetts, Willow Montana, planned to strike because Starbucks hasn’t begun bargaining with the store despite a successful union vote in April.

“If the company won’t bargain in good faith, why should we come to work where we are understaffed, underpaid and overworked?” Ms. Montana said.

Others, including a union organizer at one of the first stores to organize at Buffalo, New York, Michelle Eisen, said employees are angry that Starbucks promised higher pay and benefits to non-union stores. Starbucks says it is following the law and can’t give union stores pay hikes without bargaining.

At least 257 Starbucks stores have voted to unionize since late last year, according to the National Labor Relations Board. Fifty-seven stores have held votes where employees opted not to unionize.

Starbucks and the union have begun contract talks at 53 stores, with 13 additional sessions scheduled, Starbucks Workers United said. No agreements have been reached so far.

The process has been contentious. Earlier this week, a regional director with the labor relations board filed a request for an injunction against Starbucks in federal court, saying the company violated labor law when it fired a union organizer at Ann Arbor, Michigan. 

The regional director asked the court to direct Starbucks to reinstate the employee and stop interfering in the unionization campaign nationwide.

It was the fourth time the labor relations board has asked a federal court to intervene. In August, a federal judge ruled that Starbucks had to reinstate seven union organizers who were fired at Memphis, Tennessee. A similar case at Buffalo has yet to be decided, while a federal judge ruled against the labor relations board in a case in Phoenix.

Meanwhile, Starbucks has asked the labor relations board to temporarily suspend all union elections at its American stores, citing allegations from a board employee that regional officials improperly coordinated with union organizers. A decision in that case is pending.

Associated Press