Portuguese Prime Minister Luis Montenegro declared a "state of calamity" for the hardest-hit areas, invoking powers to mobilise more firefighters and civil servants.
He also called on police investigators to redouble their efforts to find those who started the fires and pledged help for those who have lost their homes or have been evacuated.
"We are well aware that these difficult hours are not over yet," Montenegro said in a televised address.
"We have to continue to give everything we have and ask for help from our partners and friends so that we can reinforce the protection of our people and property."
Fires have raged in Aveiro, Porto, Vila Real and Viseu. (EPA PHOTO)
The hot, dry conditions behind the outbreaks in Portugal coincided this week with flooding in central Europe. The European Union said on Wednesday the juxtaposed extreme weather phenomena are proof of a "climate breakdown".
Fellow European Union members Spain, France, Italy and Greece have committed two water-dropping aircraft each to help Portuguese firefighters.
Spain's military is also sending 240 soldiers and vehicles from its emergency response battalions specialised in combating fires across the border to its neighbour.
Montenegro made a special call for security forces to pursue both arsonists and any individuals who started a fire out of negligence.
Portuguese national police said that they have arrested seven men suspected of having started wildfires in recent days. Authorities have prohibited the use of heavy farming equipment to reduce the risk of inadvertently starting a blaze.
Among the hardest-hit areas is the district of Aveiro, south of the northern city of Porto, but several major blazes were also raging out of control in other wooded areas.
Authorities have yet to release figures for property damage or the number of evacuees, but Portuguese state broadcaster RTP has shown charred houses in rural villages and local residents trying to battle flames with buckets of water, hoses, and even large tree branches.
Three firefighters died in their vehicle on Tuesday, while another had succumbed to what authorities called a "sudden illness" while on duty over the weekend. Three civilians have also died, according to civil protection authorities.
Portugal was devastated by massive fires in 2017 that killed more than 120 people.
