Spain began three days of mourning as rescuers with drones scoured towns devastated by the floods that hit the region around the eastern city of Valencia particularly hard.

"Please, stay at home... follow the calls of the emergency services," pleaded Prime Minister Pedro Sanchez.

"Right now the most important thing is to save as many lives as possible," Sanchez told residents of the eastern Valencia and Castellon provinces.

Flags flew at half-mast on government buildings nationwide after a Mediterranean storm unleashed heavy rains and torrents of mud-filled water that swept away people, cars and homes.

Emergency services backed by more than 1,200 troops combed mud-caked towns and villages on Thursday to find survivors and clear roads of debris.

Government ministers have warned the toll is likely to rise with many people still missing and some areas remaining inaccessible to rescuers throughout Wednesday.

King Felipe VI warned the emergency was "still not over" and national weather service AEMET put parts of the Valencia region on the highest alert level for torrential rain on Thursday.

In the Valencia city suburb of Sedavi, pensioner Francisco Puente struggled to hold back the tears amid a desolate scene of upturned cars and destroyed streets.

"If you see it, you say: 'Am I seeing this? What is this?'" the 69-year-old told AFP.

Abandoned vehicles lay piled on top of each other like dominoes and some residents grabbed planks of wood to plough through the thick layers of mud, AFP journalists saw in the Valencia region.

Hundreds of people are being sheltered in temporary accommodation while road and rail transport are severely disrupted.

It could take up to three weeks to reopen the high-speed line between Madrid and Valencia, Transport Minister Oscar Puente wrote on X.

The death toll is the worst from floods in Spain since 1973 when at least 150 people were estimated to have died in the southeastern provinces of Granada, Murcia and Almeria.