Nearly 2,500 workers walked off their jobs Thursday morning from seven Marriott hotels in downtown San Francisco to demand higher wages, workplace safety and job security. Picket lines formed outside the Courtyard by Marriott Downtown, the Marriott Marquis, the Marriott Union Square, the Palace Hotel, the St. Regis, the W and the Westin St. Francis, according to Unite Here Local 2, a union that represents 89 percent of the workers.