Two female pedestrians suffered life-threatening injuries when they were hit by a van.
The 60-year-old driver was arrested after failing a roadside drugs test, said North Wales Police in a statement. The driver was released pending the results of further tests.
The crash happened on Market Street, Abergele, at about 11:00 GMT.
The women are both said to be in a serious condition at Ysbyty Glan Clwyd in Bodelwyddan.
The Welsh Ambulance Service said it sent two ambulances and a rapid response vehicle after being called to the scene near Abergele Post Office.
Sgt Emlyn Hughes, from the roads policing unit, said: "The area was quite busy around the time of the collision so we are appealing to anybody who may have witnessed the incident, or anybody who may have been travelling and who may have dash cam [footage] to come forward."