Spanish police said Monday they had arrested five people accused of trying to buy a kidney from an impoverished immigrant for €6,000 ($6,835).
Officers in Spain made the arrests working in collaboration with others in Germany and Belgium, a brief police statement said. It did not say exactly when the arrests were made.
"The immigrant tried to pull out of the deal while he was undergoing clinical tests, so he was kidnapped, beaten and threatened with death to make him go ahead," the statement said.
The person wanting to buy the organ was the leader of a criminal gang specialized in robberies, who wanted it for a son suffering from kidney disease.
Police chiefs and the head of Spain's National Transplant Organisation were due to give more details at a press conference later on Monday.
In March, 14 European nations in Spain signed the first ever international treaty to fight human organ trafficking.
The business generates $1.2 billion (1.1 billion euros) in illegal profits worldwide every year, according to the Council of Europe, which drew up the treaty.
In January 2014, Spanish police arrested a rich 62-year-old Lebanese man suspected of trying to buy the healthy liver of a Romanian.
Last year Spain carried out more organ transplants per capita than any other nation in the world.