A man has been arrested after 11 people were hurt in a series of "hit and run" car accidents.
There were "a number of casualties" in the Ely and Leckwith areas of Cardiff, said South Wales Police.
A 31-year-old man has been arrested and a van was seized.
The injured, adults and children, were taken to the city's University Hospital of Wales (UHW). Their ages and conditions were not confirmed.
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. Police advised motorists to avoid the area.
Witnesses said they saw "at least seven ambulances" and scores of police vehicles in the Ely area.
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 this 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."