Amazon's largest logistic hub in Spain was on Wednesday hit by a mass walk-out as 90 percent of its workforce took part in a two-day strike, according to a trade union spokesperson.

The logistic hub in San Fernando de Henares, in the outskirts of Madrid, was the first one opened by the United States e-commerce giant in Spain, with a staff of 1,100 plus an additional 900 part-time workers. 

The Spanish trade union Workers' Commissions (CCOO) said 70 percent of Amazon's night shift and around 90 percent of the morning shift had backed the industrial action there. 

The second-largest Spanish trade union, the General Union of Workers (UGT), said 98 percent of staff had backed the strike, which it described as an "absolute success." 

The Spanish trade unions defend upholding the current in-house agreement, seeking improved working conditions in tune with Amazon's continued expansion.

Union representatives have denounced that Amazon has tried to force its workers to accept the provincial logistic sector's standard labor agreement without offering compensation for possible losses regarding wages guarantee, overtime wages, sick-leave or job category.

For the past few months, unions and Amazon's management have been negotiating the renewal of the collective trade agreement, which was signed in 2015 and expired on Dec. 31, 2016 but was extended throughout the course of the negotiations. 

According to the trade unions, Amazon does not guarantee wage increases and intends to cheapen specialist wages, while the management pointed out Wednesday that the workforce would get a basic wage increase on Apr. 1 that could range from 1.6 to 5.6 percent per year. 

Amazon has three large logistic centers in Spain: the hub in San Fernando de Henares that opened in 2012 and two just outside Barcelona, in Martorell and El Prat de Llobregat.