Most of the victims burned to death in their cars, the Portuguese government said Sunday.
Several firefighters are among the injured, according to a BBC report.
Hundreds of firefighters and 160 vehicles were dispatched late Saturday to tackle the blaze, which broke out in the afternoon in the municipality of Pedrogao Grande before spreading fast across several fronts.
Spain dispatched two water-bombing planes on Sunday morning to aid the Portuguese fire service on the ground, said Portuguese Prime Minister Antonio Costa.
"Unfortunately this seems to be the greatest tragedy we have seen in recent years in terms of forest fires," the visibly moved Costa said.
"The priority now is to save those people who could still be in danger."
Secretary of State for the Interior Jorge Gomes earlier said that all 19 of the people then confirmed dead were civilians.
"Three of them died from smoke inhalation and 16 burned to death in their cars when they found themselves trapped by flames on the road from Figueiro dos Vinhos to Castanheira de Pera," Gomes told journalists.
Fourteen civilians were injured, 10 of them seriously, along with six firefighters, he added.
The flames spread "with great violence", moving out on four fronts, he said.
Portugal was sweltering under a severe heatwave Saturday, with temperatures exceeding 40 degrees Celsius (104 degrees Fahrenheit) in several regions.
A number of villages were affected by the blaze and evacuation procedures had been put in place for some of them, the prime minister said. Officials were not immediately able to comment on the extent of the damage.
President Marcelo Rebelo went to the Leiria region to meet families of the victims, "sharing their pain in the name of all the Portuguese people," he said.
Firefighters did "all they could" when faced with the blaze, he added.
The country was hit by a series of fires last year which devastated more than 100,000 hectares (1,000 square kilometres) of the mainland.
Fires on the tourist island of Madeira in August killed three people, while over the course of 2016 around 40 homes were destroyed and 5,400 hectares of land burned.
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