A forest is seen on fire in Pedrogao Grande. Photo: Miguel Vidal/Reuters

Many victims were burnt as they were trapped in their cars around the epicentre of the blaze in Pedrogao Grande, in what is the deadliest such disaster in Portugal's recent history.

"Portugal weeps for Pedrogao Grande," said the I newspaper while mainstream Publico's headline simply read "Why?"

"The fire has reached a level of human tragedy that we have never seen before," said a visibly moved Prime Minister Antonio Costa, who announced three days of mourning from Sunday.

Portugal's national Route 236 was transformed into an inferno as the ferocious blaze ripped through the wooded countryside.

Although the searing temperatures had dropped slightly on Monday, the fire was still raging, spreading to neighbouring regions of Castelo Branco and Coimbra.

A burned car is seen following a forest fire in Barraca da Boavista, outside Pedrogao Grande. Photo: Miguel Vidal/Reuters

Firefighters continued their grim search for bodies. Costa had warned on Sunday that the death toll could still rise.

Spain, France and Italy have sent water-bombing planes and Greece has offered firefighters. The European Union has also offered to help.

Expressing shock and horror at the deaths, UN Secretary General Antonio Guterres, who comes from Portugal, said the United Nations was ready to help.

"Our pain is immense," said Portuguese President Marcelo Rebelo de Sousa. "We feel a sense of injustice because the tragedy has hit those Portuguese of whom one speaks little – those living in an isolated rural zone."

Fire and smoke is seen on the IC8 motorway during a forest fire near Pedrogao Grande. Photo: Miguel Vidal/Reuters

'Everything burnt quickly'

Police chief Almeida Rodrigues blamed dry thunderstorms for the blaze which broke out on Saturday in Pedrogao Grande, ruling out arson. "We found the tree hit by the lightning," he said.

"Everything burnt very quickly given the strong winds. The flames passed within two or three kilometres of my house," said local resident Isabel Ferreira, 62.

"It was really hell. I thought the end of the world had come," said Maria de Fatima Nunes, a survivor.

The wooded hills in the area north of Lisbon, which 24 hours before had glowed bright green with eucalyptus and pine trees, were gutted by the flames.

Along the IC-8 highway cutting through the fire zone, smoke was still rising from the ground and small pockets of fire burned among the charred, black tree stumps.

A forest fire is seen near Pedrogao Grande in central Portugal. Photo: Miguel Vidal/Reuters

One road running through Pedrogao Grande was littered with burnt-out cars. At one spot, a police officer watched over the covered body of a victim of the fire.

'We lost everything'

Interior minister Jorge Gomes 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.

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

Another 62 people were injured, with five in a critical state including a child and four firefighters. France's foreign ministry said a French citizen was among those killed.

At a retirement home in Pedrogao Grande on Monday, about 150 people who had been evacuated or fled were waiting to learn when they could go home.

Boxes of donated food and drinks were outside the makeshift refuge, the air still reeking of smoke.

"We have people here who are waiting for news of their loved ones, who want to know and are really anxious," said Soledade Lourenco, 51, a nurse volunteering at the centre.

A forest fire is seen near Pedrogao Grande in central Portugal. Photo: Miguel Vidal/Reuters

Armindo Antonio, 67, a retiree who lives in Lisbon but has a home in nearby Louriceira, was sitting outside with a mask hanging around his neck.

"Some people stayed. Now we're waiting to see. From what they say, the village is still closed," he said.

"Near our village we have some relatives and we're trying to get in touch. We don't know if the fire got to their house, I can't reach them."

Just a few kilometres away, near Avelar, smoke was rising from a forest of pine and eucalyptus, as helicopters flying above dropped water to isolate the fire.

Over the weekend, Portugal sweltered under temperatures topping 40 degrees Celsius in several regions.

About 35 forest fires continued to burn across the country on Monday, with more than 2,000 firefighters and 700 vehicles mobilised.

Portugal was hit by a series of fires last year which devastated more than 1,000 square kilometres of the mainland.

Fires on the tourist island of Madeira in August killed three people, while across 2016 around 40 homes were destroyed and 5,400 hectares of land burned.