The downpours come a day after a similar warning for the eastern Valencia region, where more than 200 people died 11 months ago in Spain's deadliest floods in decades.

Emergency services on the islands sent a mass telephone alert to residents urging them to avoid travel and outdoor activities, and warned against approaching streams, low-lying areas or basements.

National weather agency AEMET said downpours of up to 200 litres of rain per square metre drenched Ibiza due to the "very slow" movement of the storm, the Balearic Islands' regional government said in a statement.

Footage on social media showed pedestrians wading through muddy water on an Ibiza beachfront lined with palm trees, bars and restaurants as sirens from emergency vehicles wailed in the background.

Murky brown water also gushed through streets, sweeping away large waste containers and forcing vehicles to advance at a crawl.

The Balearic government reported 132 incidents on Ibiza, most related to flooding on ground floors, roads, falling trees and urban material, as well as the risk of rivers overflowing.

Regional authorities announced the suspension of classes on Ibiza and Formentera, saying students should stay at school to avoid travel "until further notice", and ordered beaches shut.

The Spanish army's emergencies unit was being deployed with reinforcements from the neighbouring island of Mallorca and the mainland.

AEMET later downgraded its highest red alert for Ibiza and Formentera to orange after lowering a similar warning for the Valencia region.

Monday's red alert forced schools to shut for more than half a million pupils in Valencia, reviving painful memories of last year's floods.

Scientists say human-driven climate change is intensifying extreme weather events such as the heavy rainfall that triggers floods, with a warmer atmosphere capable of holding more water.

Oceans have absorbed 90 percent of the excess heat produced by human activity since the industrial age, according to scientists, causing the Mediterranean Sea to warm rapidly.