"This is a region that has had fires because of its forests, but we cannot remember a tragedy of these proportions," Valdemar Alves, the mayor of Pedróg?o Grande, was quoted as saying by the Associated Press agency.
The fires began on Saturday during an intense heatwave and rainless thunderstorms, with temperatures of more than 40C (104F) in some areas. Police say lightning striking a tree may have caused the fire.
France has offered three firefighting planes, while Spain has already sent two water-bombing aircraft to help tackle the fires. The European Union has also promised to provide firefighting aircraft.
They have sent two army battalions to help the emergency services.
Another 11 died in a village next to the motorway.
Thirty bodies were found inside cars, with another 17 next to the vehicles, on one road leading on to the IC8 motorway.
Emergency service workers were battling 156 fires across the country on Sunday, Prime Minister Costa said, adding that most of the victims had died in just one of them.Secretary of State for the Interior Jorge Gomes said that most had died from smoke inhalation and burns, while two were killed in a road accident related to the fires.
There are fears the death toll could rise, as a number of people are still missing.
Four firefighters are among the 54 people injured in the fire, which is raging in several parts of a mountainous area some 200km north-east of the capital Lisbon.
He said it was thought to have been sparked by a lightning strike.
Prime Minister Antonio Costa called it "the greatest tragedy we have seen in recent years in terms of forest fires".
Hundreds of firefighters are continuing to tackle the blaze on several fronts.
Four children are among the victims, many of whom were found dead inside their cars as they tried to flee the central forested region of Pedróg?o Grande.