More than 20 dolphins have died and several others remain stranded after swimming up a river in Cornwall, in what rescuers have described as the biggest mass stranding of marine life for 27 years.

Coastguards were alerted at 8.30am today after a visitor at a nearby guesthouse noticed a dolphin that appeared to have beached itself in Porth Creek, near Falmouth.

When they arrived to launch a rescue operation, many more dolphins had become stranded. It is thought the first dolphin may have sent out a distress signal that lured the others up the river Percuil.

"Initially, one swam up and got disoriented in shallow water," said Neil Oliver, from the Falmouth coastguard. "It put out a distress call and it looks as though the others have followed and thought 'We'll find out what's going on'.''

Divers, the fire brigade, conservationists and lifeboats were taking part in the rescue mission.

Dave Nicoll, a lifeboat helmsman, said: "It's a horrible scene of carnage with bodies everywhere, but we are doing our best to help and will continue to support the expert groups.

"We have been trying to help those who are alive and have already succeeded in getting five back into the water. We think the pod have been attracted by the cries for help from those that are stuck in the creek."

A spokeswoman for the RNLI said three volunteer lifeboat crew members had managed to help five dolphins back out to deeper water. Many dolphins were already dead when they arrived and others were struggling in the shallow waters.

Tony Woodley, the national spokesman for British Divers Marine Life Rescue, said the charity would put all of its resources into the rescue operation.

"We haven't seen a stranding anywhere near this scale since 1981 when pilot whales were beached on the east coast. This is extremely rare. We are warning people that many will die but we may be able to save some."

It is understood the creatures are striped dolphins, which are not naturally a coastal breed. Woodley said they were ocean-going and had probably followed fish that were feeding on a large algal bloom in the area.

"Logistically, a rescue like this is a minefield. It is very difficult to manage. You have to get all the dolphins together; if one or two leave the river system they will just come back to rejoin the main social group."