Emergency services said one of them was in a serious condition.
Firefighters had to be called in to extinguish the fire.
The plant belongs to Rheinmetall Expal Munitions, a subsidiary of the German arms manufacturer Rheinmetall.
It was located in Javali Viejo in the southeastern Murcia region in Spain.
The injured men, aged between 30 and 52 years old, suffered from head trauma, burns and smoke inhalation.
Emergency services said five of them were taken to hospitals in Murcia, and a sixth one was treated on site.
The factory produces artillery ammunition, mortar and medium-caliber rounds and rocket propulsion systems. Rheinmetall has not made a statement over the cause of the explosion.
The incident also sparked a forest fire in the industrial area, which spread across 2,000 square meters before being extinguished.
Rheinmetall has seven operation sites in Spain and the US and supplies weapons to over 60 countries. Ukraine is one of its biggest customers at the moment.
AzVision.az
More about: Spain Rheinmetall
