A woman was killed and 11 other people injured today in a series of "hit and run" car accidents.
The woman who was fatally injured was 32, said Superintendent Julian Williams of South Wales Police.
Four adults and seven children were hurt in the incidents in the Ely and Leckwith areas of Cardiff.
Other reports of the incident suggested the driver deliberately ploughed into parents walking home with young children after school.
"We had a customer in our shop who said the van mounted the curb like the driver wanted to run down young kids with their parents," said a female shop worker in Grand Avenue, who did not want to be named. It was really frightening, people were screaming and shouting out, it was chaotic."
Mr Williams said he could not give updates on the condition, age or sex of the injured. The hospital is not currently accepting any other admissions to accident and emergency.
The 31-year-old man was arrested near the Merrie Harrier pub in Penarth and is being held at Cardiff Bay police station. Witnesses said they saw "at least seven ambulances" and scores of police vehicles in the Ely area at the height of the incidents.
A local trader said the driver of a white vehicle "purposely" targeted pedestrians.
Ramesh Patel, who runs RM Patel Convenience Stores on Cowbridge Road West, said: "I was in the shop working at the time so I didn't actually see anything but customers have said a white van purposely drove at people. Obviously, when someone tries to run you down deliberately that is frightening, not pleasant at all."
He added: "I was working here in the store so I didn't see that myself but that is what most people who have come in here have said. People seem very shocked."