The main A470 between Rhayader and Newbridge-on-Wye is closed in both directions following a crash involving a car and a lorry.
Dyfed-Powys Police has warned the road is likely to remain closed for some time and is warning drivers to avoid the area.
Roads in Rhayader town centre are also closed today as the town is hosting a public event celebrating the arrival of the Commonwealth Games Queen’s Baton relay.
Police said they are investigating the collision which occurred at 6.20am this morning.
A spokeswoman said: "The road has been closed and will remain closed for some time as the vehicles are recovered. Drivers are advised to avoid the area."
Any witnesses to the collision are asked to contact Dyfed-Powys Police on 101.