LONDON — Demands grew on Monday for the British government to explain why it had used antiterrorism powers to detain the partner of a journalist who has written about surveillance programs based on leaks by the former National Security Agency contractor Edward J. Snowden.

David Michael Miranda, a Brazilian citizen and the partner of the American journalist Glenn Greenwald, who lives in Brazil, was held Sunday at Heathrow Airport in London for nine hours, the maximum allowed by law, before being released without charge.

“They were threatening me all the time and saying I would be put in jail if I didn’t cooperate,” Mr. Miranda said Tuesday in an interview with The Guardian newspaper, where Mr. Greenwald is a columnist. “They treated me like I was a criminal or someone about to attack the U.K.”

On its Web site, The Guardian said the interview was the first since Mr. Miranda returned to his home in Rio de Janeiro on Monday. “It was exhausting and frustrating, but I knew I wasn’t doing anything wrong,” Mr. Miranda said. Speaking separately on Monday, he said that all of his electronic equipment, including his laptop computer and cellphone, had been confiscated. In the interview, he added that he was not allowed to call his partner, who is a qualified lawyer in the United States, nor was he given an interpreter, despite being promised one because he felt uncomfortable speaking in a second language.