Some 10,000 grocery store workers across the greater Denver area went on strike Thursday, claiming unfair and illegal negotiating practices by King Soopers while their union has been negotiating a new contract with the store chain.

Striking workers at 77 King Soopers stores in Denver and its suburbs, plus those in nearby Boulder and Louisville, Colorado, urged customers not to cross picket lines that began taking shape before dawn.

“Stand together. Stay strong,” United Food and Commercial Workers International Union Local 7 President Kim Cordova wrote union members in a Monday letter announcing the strike.

UFCW Local 7 members voted by 96% last week to authorize the unfair labor practices strike.

King Soopers, a chain owned by Kroger, with 121 stores in Colorado and Wyoming, has been negotiating a new contract since October. The current contract expired in January.

The union alleges King Soopers illegally interrogated and surveilled union members, refused to provide information needed for contract negotiations, threatened union members with discipline for clothes and buttons expressing union support, and insisted on using $8 million in retiree health benefit funds to cover pay increases.

King Soopers denies all of the allegations, saying in a statement Friday it has acted in full compliance with the law and its collective bargaining obligations. Management has gone to “great lengths” to share all relevant data with the union, is committed to fair and lawful negotiations and disputes the union’s claim that it would “gut” the retiree health benefit funds.

“We want to be clear — the Union’s call for a strike is not about wages, health care, or pensions. It is based on allegations we believe lack merit,” King Soopers President Joe Kelley said in the statement.

The strike will force customers to pay higher prices at competing stores and stores with nonunion workers, Kelley added.

The strike follows several recent threatened and implemented labor union actions in the U.S. Last week, the Teamsters union and Costco reached a tentative contract agreement to avert a strike.

At Utah’s Park City ski resort, the biggest in the U.S., some 200 union ski patrollers ended an almost two-week strike Jan. 9 after reaching an agreement with resort owner Vail Resorts for higher pay including raises for senior ski patrollers.

Labor unions have secured other meaningful employer concessions in recent months following strikes by Boeing factory workers, dockworkers at East and Gulf coast ports, video game performers, and hotel and casino workers on the Las Vegas Strip.