Starbucks workers are on strike at more than 300 U.S. stores, employing more than 5,000 workers expected to strike before a five-day walkout ends later today on Christmas Eve, the union said.

Starbucks, which operates more than 10,000 company-operated stores across the U.S., said 98 percent of its stores remained open today, with about 170 stores closed.

The strike on Christmas Eve is expected to be the largest ever at the coffee chain, the Starbucks Workers United union said. “These strikes are an initial show of strength and we’re just getting started,” said one of the chain’s workers at a store in Oregon, according to the union’s statement.

Starbucks Workers United, which represents workers at 525 stores nationwide, has called for store strikes in 12 major cities, including New York, Los Angeles, Boston and Seattle, over pay, staffing and hours.

The strike began on Friday after talks between the company and the union broke down.

Starbucks declined to comment on the estimated impact on overall business from the strike, having earlier said the expected impact was “very limited”.

Earlier this month, the union rejected a proposal for non-immediate wage increases and a guarantee of a 1.5% wage increase over the next few years.

The union also said Starbucks has yet to present its workers with “a serious economic proposal.”

“We are ready to resume negotiations when the union returns to the bargaining table,” the company said.

Starbucks had previously alleged that representatives of Starbucks Workers United prematurely ended a meeting to negotiate.