The director of Madrid’s emergency services said early on Monday that firefighters had established a perimeter around the blaze and would now deploy aircraft to drop water on the flames.
“There is no risk to any population right now. We will be well prepared if the fire changes course,” Carlos Novillo said in a video posted to social media, Reuters reported.
Helicopter footage showed thick clouds of black smoke billowing up from steep scrubby slopes.
A spokeswoman for the emergency services confirmed a handful of isolated houses had been evacuated as a precautionary measure but no communities were at risk.
She could not confirm the source of the fire, which comes amid a heatwave across southern Europe that has sent temperatures soaring above 40 degrees Celsius (104 degrees Fahrenheit) in Spain, leaving it highly vulnerable.
Madrid emergency services said 16 crews as well as three aircraft were deployed to tame the blaze around the sparsely populated Robledo de Chavela region, 40km (25 miles) northwest of the Spanish capital.