The fire in the area around Viseu, southeast of Porto, broke out on Wednesday. On Saturday, authorities said the fire had so far ravaged around 10,000 hectares of land.
The fire is being fought by approximately 1,100 firefighters who are trying to bring it under control with the help of 380 vehicles and eight aircraft and helicopters.
So far, nine people have been injured, including two civilians who suffered serious injuries.
The hot weather is making the effort more difficult, with temperatures around 44 degrees in some places.
The Spanish military is assisting in the operation after Portugal requested help from the rest of Europe.
The government has introduced a ban on the public entering forest areas and restrictions on the use of certain types of machinery in rural areas, due to the risk of sparking.
