An accident has closed the A4232 in both directions from Leckwith to Junction 33 of the M4.
An eyewitness said it appeared two vehicles had been involved in the collision.
Another eyewitness reported seeing multiple police cars, fire engines, ambulances and an air ambulance.
Traffic Wales confirmed the A4232 into town is closed from B4267 Leckwith Road (Leckwith Interchange) to A48 Cowbridge Road (Culver House Cross).
They say the road has been closed in order for emergency services to assist. There is also one lane blocked on the other side.