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 and minutes of silence were observed nationwide after a Mediterranean storm unleashed heavy rains and torrents of mud-filled water that swept away people and wrecked 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.

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

In Paiporta, a suburb of Valencia city at the epicentre of the damage, 27-year-old musician David Romero lamented a "catastrophe".

"Neighbourhood after neighbourhood, street after street, there is not a business standing," he told AFP.

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.

- 'Chaos' -

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.

Scientists say human-driven climate change is increasing the length, frequency and intensity of extreme weather events.

The political fallout of the disaster started to rumble on Thursday after doubts were raised about the adequacy of warning systems.

Romero said the warnings in Paiporta only arrived when the local river was already overflowing and catching people off guard in the streets, a complaint echoed by 21-year-old Joaquin Rigon.

"Nobody warned of anything... they took out the owner of the bar here dead, drowned, chaos," Rigon told AFP.

The conservative head of the Valencia region had appeared to shift responsibility to the left-wing central government on Wednesday.

But the interior ministry criticised "erroneous information" on Thursday and said the regions, which have wide powers in Spain's decentralised political system, are responsible for managing civil protection procedures in emergencies.