The struggle took place close to a main road and over 20 minutes
A hungry bird was seen struggling to eat a 4ft long (1.2m) eel for 20 minutes before swallowing it whole on the banks of a city river.
Eyewitnesses said the eel was almost twice the size of the cormorant which was fishing in the River Taff near the Millennium Stadium in Cardiff.
One woman said the eel coiled itself round the bird's neck causing it to drop and re-catch it four times.
The RSPB said the incident proved that nature's wonders can be seen in cities.
Eyewitness Antonia Jennings, 18, said at first the cormorant versus eel battle seemed "an unfair fight".
You could see a sort of S-bend in the bird's gullet as it swallowed it alive
"The eel was bigger, more powerful and looked a lot more dangerous than a bird not much bigger than a duck," she said.
"The cormorant let go each time the eel cleverly coiled itself around its neck.
"But it caught it again four times until eventually the eel gave up.
"I couldn't believe it when the eel went down in one gulp - you could see a sort of S-bend in the bird's gullet as it swallowed it alive."
The life and death struggle happened close to a main road.
RSPB spokeswoman Delyth Willis said: "It just goes to show that some of the wonders of nature can be seen in our towns and cities - as well as out in the wilds."