MADRID (AP) — A major wildfire in northeastern Spain is raging out of control for a third straight day.

Several hundred firefighters are at the blaze in the Catalonia region Friday as Spain is forecast to endure the peak of a recent heat wave, with temperatures expected to exceed 40 degrees Celsius (104 Fahrenheit).

Officials say cooler overnight temperatures allowed crews to slow the fire's advance, but authorities say it is the region's worst fire in two decades and 20,000 hectares (50,000 acres) are at risk.

Miquel Buch, the regional interior minister, is asking people to exercise extreme caution in forested areas.

He says fire services are at full stretch "and we can't allow another wildfire to break out."

Some 500 people were evacuated to a school gym. Others were told to stay indoors.