A dual citizen of Saudi Arabia and the United States had been imprisoned in the Ritz Carlton in Riyadh for about a week when he heard a knock on his door.

Guards dragged Walid Fitaihi, a Harvard-trained physician, to another room, according to a friend who took down the prisoner’s detailed account of his treatment. Dr. Fitaihi told the friend he was slapped, blindfolded, stripped to his underwear and bound to a chair. He was shocked with electricity in what appears to have been a single session of torture that lasted about an hour.

His tormentors whipped his back so severely that he could not sleep on it for days, his friend said, speaking on condition of anonymity to avoid reprisals. The doctor had described the physical abuse, in general terms, to his relatives as well, a person close to them said.

Detained in November 2017 in what was billed as a crackdown on corruption, Dr. Fitaihi, 54, remains imprisoned without any public charges or trial. About 200 prominent Saudis were detained with him, and he is one of dozens who remain in prison.