Tourists had packed the Raganello Gorge in the Calabria region when it began to fill rapidly with rain water on Monday.
Officials say at least 11 hikers were killed when they were washed away over rocks by a raging white-water creek.
At least five people were missing but the total could be more because not all had entered the gorge with official guides or were registered.
Carlo Tansi, head of the civil protection department in Calabria said:
"This gorge filled up with water in a really short space of time and these people were catapulted out like bullets. They ended up some three kilometres down the valley,"
Tansi said the gorge was only about four metres (13 ft) wide in some places, increasing the speed of the water and making the rescue more difficult.
"It is really difficult terrain, filled with obstacles because of the (geological) formation of the area," said Eugenio Facciolla, the chief prosecutor of the provincial capital, Cosenza.
He said rescuers worked under spotlights through the night, trying to locate areas where some people may have survived by ending up on small patches of shore or tiny islands in the creek.
Rescue teams used ropes to descend the sides of the mountain to reach the site.
The nationalities of the dead and injured were not immediately known.
Six of the victims were women and five were men. Most tourists and trekkers who visit the area, in the country’s deep south, are Italian.
The Corriere della Sera newspaper said one of the injured was Dutch and had suffered a broken pelvis.
The most seriously injured were taken by helicopter to hospitals in Cosenza and Castrovillari.