An underwater search unit is being used in the hunt for a person who is thought to have fallen from a bridge into fast-flowing water on the River Dochart.
The alarm was raised at about 1530 GMT on Tuesday after several reports that someone had fallen off Dochart Bridge in Killin, Stirling.
But Central Scotland Police said "extensive searches" in the local area had so far been unsuccessful.
The person who fell was believed to be wearing a black hooded jacket.
Ch Insp Catriona Paton, who is leading the operation, said: "We don't have any further details of this person other than the hood of the top was up.
"We are keen to hear from anybody whose partner, relative or friend has failed to return home or from anybody who may have been in this area yesterday."
Ch Insp Paton said it was possible that the person concerned may have got themselves safely out of the river.
"If that is the case I urge that they contact police to let us know they are safe and well," she added.
Central Scotland Police said several witnesses had reported seeing the person falling into the water.
A helicopter, rescue boats and Killin Mountain Rescue Team have been involved in the search of the river, which flows into Loch Tay.
A spokesman for the rescue team said: "The river's high at the moment with all the rain we've had here over the past few days."
He said members of the team and specialist dogs had helped police to search the river banks on Tuesday and Wednesday morning.