Rabat - The river overflowed and submerged the pitch where an amateur football tournament was under way.
- Amazigh World News ? (@AmazighWNews) August 28, 2019
"We're in shock. I'm 64 years old and I've never seen such a downpour," the witness said.
Photographs and videos shared on social media showed muddy waters carrying away people who had clambered on top of a building flattened by the flooding.
Morocco's national weather service had warned of the risk of stormy rains on Wednesday afternoon in several provinces.
The heavy downpour followed a dry spell, making the floods more violent, local media reported.
Floods are common in Morocco. In late July, 15 people died in a landslide caused by flash floods on a road south of Marrakesh.
In 2014, floods killed around 50 people and caused considerable damage in the south of the country.
A study published in 2015 pointed to multiple failures in infrastructure maintenance, prevention, warning and emergency management.