Images posted on social media showed cars being swept along narrow streets by torrents of brown water.
At least another 12 people were still missing after the downpours, local media reported, though emergency services declined to confirm that figure.
The regional government called an emergency meeting and authorities said rescue workers and military units were heading to the area to help.
"My solidarity and support goes out to the families and friends of victims and all those affected by these tragic floods," Prime Minister Pedro Sanchez said on Twitter.
Sanchez was planning to travel to the area later today, his office said.
Lawmakers held a minute's silence before the weekly prime minister's question time this morning.
