CNN - A raging wildfire has ripped through central Portugal, killing 61 people and injuring dozens more in what officials there describe as the "greatest wildfire tragedy of recent years."
At least 54 people were injured in the blaze Saturday, including eight firefighters and a child. Four of the firefighters were in critical condition Sunday, according to Paulo Santos, an operations and emergencies official with the National Relief Operations Command in Lisbon.
The government has declared three days of mourning.
The head of Portugal's judiciary police told reporters the fire was started by natural causes.
"We have been able to determine that the origin of the fire was caused by dry thunderstorms," José Maria Almeida Rodrigues said, according to the Portuguese state-run news agency Lusa.
Some victims were burned to death in their cars as they tried to flee, officials said.
The blaze spread Saturday to the Pedrógão Grande community, forcing residents to flee the area to escape towering flames. Pedrógão Grande is about 120 miles north of Lisbon.
"Many cars could not get out and people burned to death inside their cars," Interior Ministry official Jorge Gomes told CNN affiliate TVI. Firefighters were still fighting the flames Sunday.
The municipality's Mayor, Valdemar Alves, said there were areas "completely surrounded" by the blaze and not enough firefighters to battle the flames.
"The violence of this fire was such that I am certain many people have died," Alves told TVI. "I am shocked with this death toll."
Authorities said the number of victims may rise as fire officials inspect the villages affected.
More than 700 firefighters and other emergency responders are battling the blaze.
Portuguese Prime Minister António Costa said schools in the municipalities of Pedrogão, Figueiró and Castanheira will be closed "for an indefinite period" and exams will be postponed.
European countries are coming together to assist Portugal. France and Spain have sent planes to help battle the blaze, the European Commission said.
"We express our condolences to those that have lost loved ones," said Christos Stylianides of the European Commission. "The EU is fully ready to help. All will be done to assist the authorities and people of Portugal at this time of need."
"Overwhelmed by the tragedy of Pedrógão Grande. The Portuguese people have our solidarity, support and affection," Spain's President Mariano Rajoy Brey wrote on Twitter.
Pope Francis prayed for the victims during his weekly prayer on Sunday.
"I express my closeness with the dear Portuguese people for the devastating fire that is affecting the forests around Pedrógão Grande, causing many deaths and injuries. We pray in silence," the Pope said.
Indian Prime Minister Narendra Modi also expressed his condolences on Twitter.
The Portuguese Football Federation changed its Twitter avatar to black to mourn the victims of the fire. It released a statement announcing a minute of silence to be held before Portugal's first match of the Confederations Cup against Mexico on Sunday and said it will make a donation to help the victims and their families.
Others expressed their grief on social media.
"I can't express what is going through my soul," Sérgio Fonseca posted on Facebook.
"I can't go to Figueiró dos Vinhos and my parents and brothers have had a night of horror," Fonseca said. "The roof of our home was ripped apart by the strong wind... I'm on the other side, but my head and my heart are always with them."
He later told CNN he was able to reunite with his parents and brothers, who were tired but safe and thankful to be alive.
Britain issued a travel warning for its citizens warning them of the fires in Portugal and urging them to be careful while visiting or driving through woodland areas.
"Around 2.6 million British nationals visited Portugal in 2015. Most visits are trouble-free," the British Foreign and Commonwealth office said in a statement.
On Friday, Portugal's National Authority for Public Safety issued wildfire alerts due to high temperatures, low humidity and strong winds.
Last year, thousands of firefighters battled at least 12 huge fires across the country.
Three people were killed and over 1,000 evacuated in the Portuguese island of Madeira due to the fierce wildfires a year ago.