Firefighters and water-dropping helicopters battled throughout the day to extinguish the blaze which broke out before dawn at the tyre dump near the town of Seseña, 45 kilometres (28 miles) south of Madrid.
“Everything points to the fact that this disaster was deliberate,” Seseña Mayor Carlos Velazquez told Spanish radio, adding that there had been days of rain in the area, making accidental ignition unlikely.
he government of the Castilla-La Mancha region where the dump is located said it had activated an emergency action plan as it fears the fire could last for days.
Authorities in Castilla-La Mancha warned the blaze had produced a “toxic cloud” which could affect part of Seseña, ordering the evacuation of a large apartment complex in Seseña on Friday afternoon because of the risk from the smoke.
When the evacuation order was given there were only about 1,000 people still in the complex — the other 8,000 residents had already left voluntarily.
Local officials provided eight buses to help transports residents who did not have cars and provided 600 beds in local sports centres for those who did not have a place to stay.
“We have decided to evacuate part of the population as a precaution in case there is a change in weather conditions. If the weather improves we hope people can return to their homes tomorrow,” Juan Ruiz Molina, the regional government’s public administration minister, told a news conference.
Doctors interviewed on Spanish television said inhaling large amounts of the smoke could cause chemical pneumonia, asthma and eye, nose and throat irritation, especially in children, old people and those with weak respiratory systems
Some area schools stayed closed for the day. There are no reports of any injuries.
The tyre dump stretches over 10 hectares (25 acres), the equivalent of about 10 football pitches, and straddles the Castilla-La Mancha and Madrid regions.
About 70 percent of the tyres had burned by Friday night but officials did not know when the blaze would be completely extinguished, said Francisco Martinez, the regional government’s environmental minister.
Luis Villarroel, a Madrid firefighting official at the scene, said firebreaks that were created helped restrict the blaze to one active front and it was gradually being brought under control.
The billowing black smoke emanating from the blaze was visible from the Spanish capital but Madrid city hall said on Friday evening that the city’s air quality had not registered any change from the fire.
The massive stack of tyres started to form in the 1990s when a company began using the site as a temporary depot for old tyres due to be recycled.
But over the years these started to accumulate, resulting in three-metre (10-feet) high piles.
At the end of 2015, Madrid and Castilla-La Mancha invited bids to empty the dump and destroy the tyres, but the process has yet to start.