The fire broke out on Saturday in the municipality of Pedrogao Grande in central Portugal, before spreading fast across several fronts.
On Sunday afternoon, nearly 900 firefighters and 300 vehicles were still battling the blaze as scenes of devastation could be seen around the town.
"Unfortunately, this seems to be the greatest tragedy we have seen in recent years in terms of forest fires," said a visibly moved Prime Minister Antonio Costa, who declared three days of mourning starting on Sunday.
The flowing expanse of hills situated between Pedrogao Grande, Figueiro do Vinhos to the west and Castanheira de Pera to the north, which 24 hours before had glowed bright green with eucalyptus plants and pine trees, were completely gutted by the flames.
A thick layer of white smoke hovered over either side of a national motorway for a distance of about 20 kilometres (12 miles), as blackened trees leaned listlessly over charred soil.
A burnt-out car sat outside partly destroyed and abandoned houses, while a few metres away police in face masks surrounded the corpse of a man hidden under a white sheet.
Secretary of State for the Interior Jorge Gomes said 62 people burned to death, mostly trapped in their cars engulfed by flames in the Leiria region.
"It is difficult to say if they were fleeing the flames or were taken by surprise," he said.
More than 50 people were injured, five critically, including one child and four firefighters.
"The number of fatalities could still rise," Costa said. "The priority now is to save those people who could still be in danger."
'Fire raging on four fronts'
The European Union said it would provide firefighting planes following a request from Lisbon.
"France has offered three planes through the EU Civil Protection Mechanism and they will be quickly sent to assist the local emergency efforts," EU Commissioner for Humanitarian Aid and Crisis Management Christos Stylianides said.
Portugal was sweltering under a severe heatwave over the weekend, with temperatures topping 40 degrees Celsius (104 degrees Fahrenheit) in several regions.
About 60 forest fires broke out across the country during the night, with around 1,700 firefighters battling to put them out.
"The fire is still raging on four fronts," Gomes said, two spreading "violently".
Dry thunderstorms were thought to have been the cause, according to the prime minister.
A number of villages were affected by the main fire and homes were evacuated. Some were sheltered in neighbouring areas.
Officials were not immediately able to comment on the extent of the damage.
Spain dispatched two water-bombing planes on Sunday to aid the Portuguese fire service, Costa said.
'Didn't want to die in their homes'
Dozens of people who fled their homes were taken in by residents of the nearby municipality of Ansiao.
"There are people who arrived saying they didn't want to die in their homes, which were surrounded by flames," Ansiao resident Ricardo Tristao told reporters.
President Marcelo Rebelo went to the Leiria region to meet families of the victims, saying he was "sharing their pain in the name of all the Portuguese people".
Firefighters did "all they could" when faced with the blaze, he said.
Pope Francis began his Angelus prayer by invoking the tragedy.
"I express my closeness to the beloved people of Portugal following the devastating fire," Francis said. "Let's pray in silence".
Portugal 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.