Police and fire crews are at the scene of multiple-vehicle crash on junction 33 of the M4 near Cardiff, which is believed to involve a coach and several other vehicles.
A police spokesman could not confirm if the crashes were related. He said there were a number of seriously injured people and walking wounded.
All the emergency services are at the scene, including the air ambulance.
The crash involves two cars and a coach on the M4 eastbound and a car had been overturned on the other side of the road, an eyewitness told WalesOnline.
A spokesman for South Wales police said: "We have had multiple calls and we are dealing with it as we speak. That's all the information I can give at the moment but we are aware."
A South Wales fire spokeswoman said: "We have just booked an attendance to it just now, but that is all the information we have at the moment."
Three lanes out of four eastbound lanes are closed, between J34 A4119 (Miskin) and J33 A4232 (Cardiff West And Services).
Initially, eastbound traffic was stopped for five minutes. Lanes one, two and three (of four) remain closed with emergency services in attendance. On the westbound side one lane is closed.
Also, the M4 at J34 A4119 (Miskin) eastbound entry slip road is closed, to prevent traffic joining and increasing congestion caused by the collision.