The westbound carriageway between junction 33 at Capel Llanilltern and junction 34 at Miskin was shut at 3.51pm on Monday following a collision.
An air ambulance was reported to have been called to the scene, and one person was seen lying on the floor, according to an eyewitness.
Eastbound traffic was also impacted, with lane closures in place.
Motorists were diverted via the exit and entry slip roads.
Traffic Wales South said to expect long delays.
In a statement posted online, the agency said: “Westbound, J33 Capel Llanilltern to J34 Miskin -Traffic is being diverted via the off and on-slip.
“Eastbound, J34 Miskyn to J33 Capel Llanilltern is now closed.
“This is a serious incident and we expect the road to remain closed for sometime.”
AA Traffic Cameras showed heavy congestion stretching back to almost junction 28.
Traffic Wales South first reported the closure at 3.51pm.
Further information will be published as it becomes available.
More to follow.
