The Spanish army's emergency response unit said it had deployed overnight near the northeastern city of Tarragona to assist local authorities.
The blaze has devoured "around 2,377 hectares of land, mostly forest", countryside rangers in the Catalonia region wrote on X.
The protected Els Ports natural park makes up 30 percent of the affected area, they added.
Firefighters released video footage shot from a helicopter on Monday showing hills enveloped in a cloud of grey and orange smoke stretching into the distance.
Winds of up to 90 kilometres per hour (56 miles per hour) made it harder to extinguish the fire overnight, firefighters said.
The national civil protection authority urged residents in the area to close their doors and windows and stay home, saying that around 18,000 people were affected.
Scientists say human-induced climate change is increasing the intensity, length and frequency of the extreme heat that causes some forest fires.
Spain has in recent days sweltered through a heatwave that parched the land and heightened the risk of forest fires.
National weather agency AEMET said last month was Spain's hottest June on record and that the frequency of extreme heat had tripled in the past 10 years.
According to the European Forest Fire Information System (EFFIS), some 500 fires destroyed 300,000 hectares in Spain in 2022, a record for the continent.
Around 21,000 hectares have burned so far this year.
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Forest and Wild Fires - News, Science and Technology
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