Emergency services said that two of the injured were being treated for major burns.
The civil protection agency in the Catalonia region called the blast a “chemical accident” and initially warned people in parts of the city of 800,000 and in nearby towns to refrain from going outside as preventive measures.
The confinement measures were lifted and two roads that had been cordoned off were reopened some three hours after the evening blast.
Interior department chief Miquel Buch tweeted that the fire was not toxic but he advised residents to remain inside.
The blast took place on the premises of Iqoxe, according to a woman who answered the phone at the company and who quickly hung up the phone without giving her name.
According to the company’s website, Iqoxe is Spain’s sole producer of ethylene oxide, a chemical compound used for making detergents, solvents and other products, as well as glycol, one of the main raw materials in the production of plastics.
Local residents posted videos showing the aftermath of the blast, with flames and a big column of black smoke emerging from an area dotted with big industrial tanks.
Fire departments sent 29 brigades to the area, the regional emergency service said. It said firefighters were inspecting the complex´s building structures as well, trying to fully extinguish the fire in one ethylene oxide tank.
Some local residents told Tarragona Radio that the blast could be heard from miles away.
Tarragona is located 115 kilometers (71 miles) southwest of Barcelona, the regional capital of Spain’s Catalonia region.
Since 2015, the city has housed a 1,200-hectare (2,965-acre) “chemical hub,” ChemMed, that was described as the largest of its kind in southern Europe.
The government said Prime Minister Pedro Sánchez had been in contact with the regional president of Catalonia, Quim Torra to inquire about the blast and offer any help needed.
Tuesday’s explosion comes just over a month after a Dec. 11 blaze at a solvent and industrial residues recycling plant in the northeastern town of Montornes del Valles. After that blast, which caused no casualties, authorities had to cordon off an area of some 500 meters (550 yards) in radius around the plant.