Over 20 rare striped dolphins have died after becoming stranded in a creek near St Mawes.
RNLI crews battled for hours to save the remaining dolphins, which were stuck in shallow water and on the shoreline at Porth Creek on the Percuil River.
The Falmouth RNLI Atlantic 75 inshore lifeboat was on the scene this morning and crew members have described what they saw as "carnage".
By the time the volunteer crews arrived more than 15 of the mammals were already dead.
Rescuers managed to shepherd seven of the stricken animals back out into deeper water to rejoin their waiting pod.
RNLI helmsman on the scene, Dave Nicoll, said his two crew had been in the water with the dolphins.
"We have been trying to help those who are alive.
"They swam out towards the pod of dolphins that have gathered in the river and we hope they will be all right."
He said they thought the pod had been attracted by the cries for help from those stuck in the creek.
"It is a horrible scene of carnage with bodies everywhere, but we are doing our best to help and will continue to support the expert groups when they arrive," he added.
Members of the British Divers Marine Life Rescue and a local vet were at the scene.
A spokesman said this was the largest mass stranding along the UK coast for many years.
It is thought the dolphins may have been hunting for fish which had been feeding on algae in the river mouth.
The dead dolphins have been removed from the creek for post morten examination after beinglogged by Cornwall Wildlife Trust.
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