Hundreds of firefighters on Thursday battled an out-of-control wildfire in Spain that authorities believe started after a heap of manure self-ignited amid a European heatwave.

The flames raging across a hilly area roughly 80 kilometers (50 miles) inland from the Mediterranean coastal town of Tarragona could eventually devour 20,000 hectares in what was presented as an "extreme risk," said a statement.

The blaze, which broke out Wednesday afternoon in Torre del Espanol in the northeastern region of Catalonia, has so far affected more than 6,500 hectares (16,000 acres), according to the regional government, an area about 19 times the size of Central Park in New York.

Some 350 firefighters backed by around 230 soldiers and 15 aerial tanker aircraft were at the scene of the blaze, the worst in Catalonia in 20 years in terms of the amount of land charred.

The fire raged several kilometers from the Asco nuclear plant but officials said the site was not at risk since winds were blowing the flames away from it and it is located on the other side of a river.

Firefighters said that steep terrain was hampering efforts to control the blaze, which spread quickly due to strong winds and soaring temperatures which neared 40 degrees Celsius (104 Fahrenheit). 

"It's complicated. We won't get it stabilized today," said regional firefighting chief Manuel Pardo when speaking to local public television.

Critical situation

Catalonia's forest agent service said the fire likely began when an "improperly managed" pile of manure at a chicken farm self-combusted in the extreme heat.

Around 50 people have been evacuated from their homes and five roads have been shut, the regional government said. Many evacuees told local Spanish media that they fled with just the clothes on their backs.

Emergency services ordered the roughly 250 residents of the village of Bovera to stay indoors and keep their windows shut to avoid inhaling smoke.

The charred land includes vineyards, olive groves and fruit trees. A farm burned down at Torre del Espanol, killing more than 200 sheep and at least two horses, according to media reports. 

Catalan regional president Quim Torra warned that Friday was expected to be the "hottest day" of the heatwave "and the situation will be critical" in all of Catalonia.

He announced that harvesting crops would be forbidden in Catalonia to avoid accidentally sparking fires and access to mountains would be restricted as a precaution as long as the high temperatures continue.

Meteorologists blame a blast of hot air from northern Africa for scorching temperatures early in the European summer, which could send thermometers towards 40 degrees Celsius in France, Spain and Greece on Friday.