The deadliest flash floods in Spain's modern history have killed at least 214 people and dozens were still unaccounted for, four days after torrential rains swept the eastern region of Valencia, Prime Minister Pedro Sanchez said on Saturday.

In a televised statement, Sanchez said the government was sending 5,000 more army troops to help with the searches and clean-up in addition to 2,500 soldiers already deployed.

"It is the biggest operation by the Armed Forces in Spain in peacetime," Sanchez said. "The government is going to mobilize all the resources necessary as long as they are needed."

Valencian regional authorities said on Saturday night the total number of fatalities in the region was 211, plus two from Castilla La Mancha and one in Andalusia.

The tragedy is already Europe's worst flood-related disaster since 1967 when at least 500 people died in Portugal.

Hopes of finding survivors were raised when rescuers found a woman alive after three days trapped in a car park in Montcada, Valencia. Residents burst into applause when civil protection chief Martin Perez announced the news.

Volunteers flocked to Valencia's City of Arts and Sciences centre on Saturday for the first coordinated clean-up organised by regional authorities. The venue has been turned into the nerve centre for the operation.

In Valencia's Picanya suburb, shop-owner Emilia, 74, told Reuters on Saturday: "We feel abandoned, there are many people who need help. It is not only my house, it's all the houses and we are throwing away furniture, we are throwing away everything.

"When is the help going to come to have fridges and washing machines? Because we can't even wash our clothes and we can't even have a shower."

Nurse Maria Jose Gilabert, 52, who also lives in Picanya, said: "We are devastated because there is not much light to be seen here at the moment, not because they are not coming to help, they are coming from all over Spain, but because it will be a long time before this becomes a habitable area again."

The storm triggered a new weather alert in the Balearic Islands, Catalonia and Valencia, where rains are expected to continue during the weekend.

Scientists say extreme weather events are becoming more frequent in Europe, and elsewhere, due to climate change. Meteorologists think the warming of the Mediterranean, which increases water evaporation, plays a key role in making torrential rains more severe.