Amazon Prime Day threatened by strikes at European warehouses over working conditions

Amazon workers in Spain went on strike Monday to protest working conditions at the e-commerce giant's warehouses, just as its massive Prime Day sales kick off.

A group called Amazon En Lucha organized the walk out at the company's fulfillment center just outside the Spanish capital of Madrid. The strike will last until July 18 and the employees also called for Amazon workers around Europe to go on strike.

Those on strike stood outside the warehouse, which was Amazon's first in Spain, wore masks of Chief Executive Jeff Bezos. It's not the first time employees at the fulfillment center have gone on strike this year. In March, hundreds of workers walked out in a bid to negotiate over working contracts.

Their specific grievances include an increase in working hours, the elimination of bonuses and lack of protection against illnesses, according to a statement released on Sunday by Spain's communist party.

It's unclear how many people actually went on strike. Amazon En Lucha tweeted a picture showing the number of empty parking spots at the warehouse. They also called for a Europe-wide walkout, but again it's not clear how many Amazon workers across Europe were involved. Fisacat, a workers union in Italy, threw its support behind the Spanish workers in a tweet on Monday.

An Amazon spokesperson told CNBC that it is a "fair and responsible" employer.

"Amazon’s total compensation in Madrid is in the high range of the logistics sector and consists of base pay and an extensive benefits package: private medical insurance, a company pension plan, life assurance, employee discount and a Career Choice program that provides employees funding for adult education, offering to pre-pay 95 percent of tuition and associated fees for nationally recognized courses, over four years," the spokesperson said.

"Amazon has already invested over 1.1 billion euros ($1.29 billion) in Spain and created over 2,000 permanent jobs since 2011 and we continue to be committed to Spain."

Prime Day is the e-commerce giant's annual event where it discounts many products. But Amazon has come under heavy fire from workers across Europe. In November, workers in both Germany and Italy went on strike over pay and working conditions.

The U.S. company's warehouse conditions have come under scrutiny in the past few years. An investigation by the U.K.'s Mirror newspaper outlined how workers had timed toilet breaks and strict targets to meet with many falling asleep on the warehouse floor. A British union found out via a Freedom of Information request, that ambulances have been called out 600 times to Amazon's U.K. warehouses in the past three years. At the time, Amazon said that it was “simply not correct to suggest that we have unsafe working conditions based on this data or on unsubstantiated anecdotes."

It's unlikely that the strikes in Spain will cause much disruption to Amazon Prime Day which continues to be a key event for the company. Last year, it said that Prime Day was its "biggest day ever" with sales growing by more than 60 percent from the same period in 2016. The event is key for Amazon to lure more people onto its $119 a year Prime subscription service.