An 80 tonne crane is upside down in a field in Builth Wells, Powys, leaking diesel and hydraulic fluid, emergency services have said.
Mid and West Wales Fire Service was called to the scene, off the A470 near Erwood Bridge at around 17:50 GMT.
There are no reports of any injuries but there are concerns about pollution as it is very close to the River Wye.
A recovery operation is ongoing and an environmental unit is being brought in to try to prevent pollution spreading.
Natural Resources Wales has also been contacted.
Dyfed-Powys Police said traffic in the area is facing delays.
One lane has been closed and a stop/go system is being used to divert motorists.