More than 1,500 firefighters in Portugal are still battling to control the deadly wildfire that raged across a central region of the country over the weekend, killing at least 62 people and injuring dozens more.

The government has declared three days of national mourning as it awaits the arrival of more water-dropping planes from Spain, France and Italy. Both the EU and the UN have pledged to provide any necessary assistance.

The fire, which broke out on Saturday in the municipality of Pedrógão Grande and is thought to have been caused by a lightning strike, quickly spread in several directions.

Many of the victims died in their cars as they tried to flee the flames. More than 60 people were injured, with 18 of them taken to hospitals in Lisbon, Porto and Coimbra. Five of them – four firefighters and a child – are in a critical condition.

Portugal’s prime minister, António Costa, said the death toll was likely to rise as the emergency services conducted village-to-village searches, adding: “The fire has reached a level of human tragedy that we have never seen before.”

António Guterres, the former Portuguese prime minister who is now the UN secretary general, said he was shocked and horrified by the fires. “The United Nations stands ready to assist in any way possible,” he said.

The wooded hills of Pedrógão Grande, 90 miles (150km) north of Lisbon, were gutted by the flames.

Above the blackened trees and charred soil, a thick layer of white smoke hovered over either side of a motorway for about 12 miles. A burnt-out car sat outside partly destroyed and abandoned houses, while a few metres away police in face masks surrounded the body of a man hidden under a white sheet.

Police chief Almeida Rodrigues ruled out arson, saying a tree had been struck by lightning during one of the dry thunderstorms that have punctuated the current heatwave.

The Iberian peninsula is sweltering under unseasonably hot weather, with temperatures rising past 40C (104F) in some regions.

“Everything burnt very quickly given the strong winds,” said local resident Isabel Ferreira, 62, who saw the flames pass within two miles of her house. “I knew several of the victims. One of my colleagues lost her mother and her four-year-old girl as she could not get them out of the back of the car.”

Jorge Gomes, Portugal’s secretary of state for internal administration, said 18 of those burned to death had been trapped in their cars engulfed by flames on the road between Figueiro dos Vinhos and Castanheira de Pera. “It is difficult to say if they were fleeing the flames or were taken by surprise,” he said.

Other bodies were found in houses in isolated areas. At least three villages near Pedrógão Grande were evacuated.

Luisilda Malheiro and her husband Eduardo Abreu, a couple of farmers, both 62, escaped the inferno on route N-236. “We escaped in time, me on the tractor and he with our van,” Malheiro said. “Our house is still there but we lost everything else: the chickens, the rabbits and the ducks. We were only able to save two goats.”

Valdemar Alves, the local mayor, said he was stunned by the number of deaths. “This is a region that has had fires because of its forests, but we cannot remember a tragedy of these proportions,” he said.

More than 140 forest fires continued to burn across the country on Sunday night, with about 2,000 firefighters battling to put them out.

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,” said Ansiao resident Ricardo Tristao.

President Marcelo Rebelo went to the Leiria region to meet victims’ families, saying he was “sharing their pain in the name of all the Portuguese people”.

Pope Francis offered his condolences in his Sunday prayers while EU commission chief Jean-Claude Juncker hailed the bravery of the firefighters.

Portugal was hit by a series of fires last year which devastated more than 100,000 hectares (247,000 acres) of the mainland.

Fires on the island of Madeira in August killed three people, while across 2016 about 40 homes were destroyed and 5,400 hectares of land burned. In 1966, a blaze in the forest of Sintra, west of Lisbon, killed 25 soldiers trying to battle the flames.