The man, who has not been named, convinced patients to stop their treatments and use his alternative methods instead, according to a police statement.
A police spokesman told CNN the man would approach people with terminal or late-stage illnesses.
Three of his patients -- aged 14, 16 and 76 -- subsequently died.
At least 45 people fell for the man's deception, police said. He was arrested in Athens on Tuesday under a warrant for homicide, fraud, counterfeiting and drug trafficking and was handed over to prosecutors.
Police said they investigated the man for more than a year and discovered he was pretending to be a neurologist, a neurosurgeon and a physiotherapist, as well as a US-educated researcher into pediatric surgery and pediatric oncology at a major medical center in Switzerland.
He also pretended to be a member of several well-known Greek families, an executive of the International Committee of the Red Cross, an air force pilot and part of a US investment fund.
The man promised to treat people with serious illnesses and gave them "substances of unknown origin and chemical composition, without medical indications," police said. Some of the substances contained cannabis, they added.
He would accompany patients to medical facilities for examinations, or coordinate surgeries, and took large sums of money from them.
In one case, he received at least €15,000 ($16,800), as well as more than €58,000 from the three patients who died, police said.
They said the man had three accomplices, adding that they are investigating the possible involvement of more people.
They confiscated two airguns, two knives, doctor ID stickers, notes on diet programs, herbs and CDs containing information on patient examinations.
Police have appealed to anyone with information on the case to come forward.
On Thursday, Giorgos Patoulis, president of the Athens Medical Association, called for closer monitoring of the authenticity of medical diplomas.