Seven horses running loose on the carriageway have caused problems for motorists on the westbound side of the A48 (M) in Cardiff.
Gwent Police officers were called to the scene at 0601 BST on Thursday.
The horses were initially spotted on the A48 Tredegar Park roundabout but then moved on to the nearby road linking to the M4 motorway.
Police are slowly trying to encourage the animals to leave the road and have set up rolling roadblocks.
Traffic going eastbound is unaffected.
It is not known where the horses have come from.