HONG KONG — Blindfolded and handcuffed, the bookseller was abducted from Hong Kong’s border with mainland China and taken to a cell, where he would spend five months in solitary confinement, watched 24 hours a day by a battery of Chinese guards.

Even the simple act of brushing his teeth was monitored by minders, who tied a string to his toothbrush for fear he might try to use it to harm himself. They wanted him to identify anonymous authors and turn over data on customers.

“I couldn’t call my family,” the man, Lam Wing-kee, said on Thursday. “I could only look up to the sky, all alone.”

Months after he and four other booksellers disappeared from Hong Kong and Thailand, prompting international concern over what critics called a brazen act of extralegal abduction, Mr. Lam stood before a bank of television cameras in Hong Kong and revealed the harrowing details of his time in detention.