The accident happened at around 11 am (0900 GMT) while several workers were dismantling port cranes, a local emergency services spokeswoman told AFP.
Two mobile cranes which were being used to dismantle a stationary crane "gave way" for yet to be determined reasons "dragging down the six workers involved in the work", Gijon's firefighting force wrote on social network X.
Two workers aged 24 and 49 died in the accident while four others were injured, two of them seriously, it added.
Labour Minster Yolanda Diaz said she regretted the deaths and wished the injured "a speedy recovery".
"No one should put their life at risk at work. It is necessary to continue to strengthen occupational risk prevention so that this does not happen again," she added on X.