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, Superintendent Julian Williams of South Wales Police said.
Four adults and seven children were hurt in the incidents in the Ely and Leckwith areas of Cardiff.
The injured were taken to the city's University Hospital of Wales.
Williams said that a 31-year-old man in a flat bed truck had been arrested on suspicion of murder.
Early reports indicated there were around five different "hit and runs", and officers attended a number of locations including Crossways Road, Cowbridge Road West, Grand Avenue, and Sloper Road.
Williams said the fatality occurred at Cowbridge Road West, outside the fire station.
"Our thoughts are obviously with her family," he said.
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," a female shop worker in Grand Avenue, who did not want to be named, said.
"It was really frightening, people were screaming and shouting out, it was chaotic," she added.
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."
Another trader in nearby Grand Avenue said customers were shocked and panicked by what had happened.
Harbhajan Dhaliwal, 42, runs a general and convenience food store in the road.
"People are very upset and panicked by what has happened. They are very shocked," he said. "I had one woman come in who said her niece has been injured. She is really upset and worried and was going to go to the hospital.
"People can't understand what's happened, some say the driver was drunk, others think he had some kind of breakdown at the wheel. Customers have said he was targeting people as they walked along.
"People coming in here are very very shocked and panicked. It is actually quite quiet here now. Business is slower than usual because the road is closed after what has happened. This has also upset people."
A Welsh Ambulance Service spokesman said: "The Welsh Ambulance Service has responded to a number of road traffic collisions in the Ely and Leckwith areas of Cardiff on Friday afternoon.
"Eight emergency ambulances, five rapid response vehicles, four ambulance officers, two basics doctors and one Helimed attended several calls along Crossways Road, Cowbridge Road West, Grand Avenue and Sloper Road.
"Twelve casualties, including adults and children, have been taken to the University Hospital of Wales with various injuries, some serious.
"The A&E department at UHW has been closed to anything other than casualties related to these incidents. In the meantime, the ambulance service is liaising with hospitals in the region including The Royal Gwent and The Royal Glamorgan to convey emergency patients."
Williams also told reporters: "Whether the actions were deliberate or reckless is a matter for the inquiry and the person will obviously be spoken to."
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