A smoke rises above trees during a forest fire in Pedrogao Grande. Some 600 firefighters were trying to put out the fires, which started Saturday.
Prime Minister Antonio Costa called it "the biggest tragedy of human life that we have known in years".
"The priority now is to save those people who could still be in danger".
"The violence of this fire was such that I am certain many people have died", Alves told TVI.More news: Thomas shines as Harman takes US Open lead
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Secretary of State for the Interior Jorge Gomes earlier said all 19 of the people then confirmed dead were civilians. At least 16 people were injured, including five firefighters. Fourteen of the injured were in serious condition, RTP said.
Portugal was coping with a severe heatwave on Saturday, with temperatures exceeding 40C in several regions.
He also said a period of national mourning would be announced shortly. Officials were not immediately able to comment on the extent of the damage. Around 500 firefighters in 160 vehicles were later dispatched to tackle the blaze. It is prone to forest fires in the dry summer months.
Fires on the tourist island of Madeira in August killed three people, while over the course of 2016 around 40 homes were destroyed.