The government has declared a state of emergency in all the regions north of the Tagus river, a territory about half the size of continental Portugal. Schools were closed Monday and at least 20 planes were joining 350 firefighting units in tackling the blazes.
Arsonists are being blamed for causing most of the 105 fires in the Galicia region of northwest Spain, which borders Portugal.
Meanwhile, Interior Minister Juan Ignacio Zoido also said that several people had already been identified in connection with the fires, and has appealed for anyone with further information to share it with the national protection service.
Three people died in Spain's Galicia region - two of them women found inside a burnt-out vehicle, the third a man in his 70s, killed as he tried to save his farm animals.
"They are absolutely intentional fires, premeditated, caused by people who knew what they are doing", said Alberto Nunez Feijoo, the head of the Galicia government. Solidarity of all Spain and prompt coordination to combat the fire and assist people'.
Agency spokeswoman Patricia Gaspar said the death toll could rise: "We are still searching burnt areas to see if there are any more victims".
Spanish TV pictures showed people trying to fight fires with buckets of water.
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The city of around 300,000 residents has opened up two sports centres and booked rooms in three hotels for people who had to evacuate their homes.
Two people were killed when they were trapped by flames in a van in Chandebrito, northwestern Spain.
"We went through absolute hell, it was frightful".
The 524 registered outbreaks of fire in Portugal, by far the most since 2006, were caused by "higher than average temperatures for the season and the cumulative effect of drought", Gaspar said. "There was fire everywhere", a resident of the town of Penacova, near Lousa. told RTP television.
The situation was still "critical" because of unseasonably high temperatures.
Portugal's Coimbra, Castelo Branco, Aveiro, and Viseu regions were hardest hit as the fires continued into Monday.
Officials in both countries said they expected that rain and cooler weather forecast for later on Monday would help put out the fires.