SÃO PAULO—Subway workers in this World Cup host city backed down from a threatened strike on the opening day of the soccer tournament, removing a cloud from global event whose preparations have been plagued by delays and protests.

Union members voted Wednesday to continue working despite its threats to walk off the job unless 42 co-workers fired this week were reinstated.

São Paulo Metro workers' union President Altino de Melo Prazeres Júnior said members were worried about a potential public backlash. "What weighed on our decision was fatigue and the fear of some workers that people could view our decision as a move to disturb the World Cup," he said.

The union's retreat defuses a potential public-relations nightmare for Brazil, which has struggled with delayed stadiums, unfinished infrastructure and protesters angry over the tournament's cost.

The Metro is a vital means of transport in this traffic-choked city and a means to shuttle thousands of World Cup visitors to São Paulo's main stadium, about 13 miles from the center. The tournament begins on Thursday and lasts a month.