Raging forest fires in central Portugal killed at least 61 people, many of them dying because they were trapped in their cars when flames swept over a road.

Portugal declared three days of national mourning on Sunday as Portuguese Prime Minister Antonio Costa called the disaster "the biggest tragedy of human life that we have known in years."

Interior Ministry official Jorge Gomes told state broadcaster RTP that the number of victims from the flames and smoke inhalation has reached 60, while another two people perished in a traffic accident related to the fires.

The death toll was earlier on Sunday reported at 62 people, but Portugal Prime Minister Antonio Costa later reduced it to 61 because authorities allegedly counted one victim twice.

The fatalities occurred in the Pedrogao Grande area, about 95 miles northeast of Lisbon, where about 600 firefighters have been trying to put out the fires since Saturday, Gomes said.

A wildfire is reflected in a stream at Penela, Coimbra, central Portugal. PATRICIA DE MELO MOREIRA / AFP - Getty Images

Gomes said that at least 16 people were killed when their vehicles were engulfed by flames on a road between the towns of Figueiro dos Vinhos and Castanheira de Pera.

A lightning strike is believed to have sparked the blaze after investigators found a tree that was hit during a "dry thunderstorm," the head of the national judicial police told Portuguese media.

Related: Raging Forest Fire Kills at Least 61 in Portugal

Dry thunderstorms are frequent when falling water evaporates before reaching the ground because of high temperatures. Portugal, like most southern European countries, is prone to forest fires in the dry summer months.

A huge wall of thick smoke and bright red flames towered over the top of trees near houses in the wooded region.

A burnt car on a road after a wildfire in Pedrogao, in central Portugal. PATRICIA DE MELO MOREIRA / AFP - Getty Images

Local resident Isabel Brandao told The Associated Press that she had feared for her life.

"Yesterday we saw the fire but thought it was very far. I never thought it would come to this side," she said. "At 3:30 a.m., my mother-in-law woke me up quickly and we never went to sleep again. We were afraid the fire would reach us."

Spanish state television showed terrifying images from Portugal of several people on a road trying to escape the intense smoke that had reduced visibility to a question of a few yards.

Costa said that firefighting crews were having difficulties in approaching the area because the fire was "very intense." He added that Portuguese authorities were working on identifying the victims and that Spanish rescuers would assist in efforts to control the blazes.

Firefighters work to put out a forest fire near Bouca, in central Portugal. RAFAEL MARCHANTE / Reuters

Costa said that while investigations are ongoing to determine the cause of the fire, authorities believe that the high temperatures of 104 F in recent days may have played a part.

"This is a region that has had fires because of its forests, but we cannot remember a tragedy of these proportions," said Valdemar Alves, mayor of Pedrogao Grande. "I am completely stunned by the number of deaths."

Spain's prime minister has offered support to its neighboring country. Mariano Rajoy has tweeted that he is "overwhelmed by the tragedy at Pedrogao Grande. The Portuguese people can count on our solidarity, support and care."