The humpback whale which beached itself at the Van Staden's River mouth, 40km west of Port Elizabeth late on Sunday, was blown up on the beach on Monday.
"The police carried out a demolition-controlled explosion on instructions from Marine and Coastal Management and death was instaneous," research assistant at the Bayworld Aquarium Wendy Kant said.
"It was not pleasant, it was horrible to see but it was the humane thing to do."
Staff from Bayworld Aquarium, the National Sea Rescue institute, metro ambulance service, surf-rescue lifesaving clubs and volunteers on Sunday battled for more than nine hours to return the whale to the sea.
Kant said the whale had been lying on the beach for such a long time and had already suffered internal damage.
"The whale was still alive on Monday morning but was weak and barely breathing," Kant said.
It was lying on the sand, completely exposed and had overheated. Its internal organs had been severely damaged and the lungs had partially collapsed.
"The animal was clearly in great pain and the decision was taken by (Marine and Coastal Management) on Monday to end its suffering."
Kant said methods usually used included injecting toxic liquid directly into the mammal's heart or veins, slicing a main artery and letting it bleed to death or rigging an explosion near the animal's head.
She said the size of the whale necessitated an explosion.
Members of the public who were on the scene were briefed on the whale's condition and how it would be put down before they were removed from the site.
Measurements were taken of the 10m animal and samples of parasites on its body were taken for research purposes.
The carcass is expected to remain on the beach until the high tide takes it out to sea. - Sapa