David Miranda - the Brazilian partner of Glenn Greenwald, an American journalist with Britain's Guardian newspaper - was held for almost nine hours on Sunday as he passed through London's Heathrow Airport on his way to Rio de Janeiro from Berlin.
A furious Greenwald said British authorities had "zero suspicion" that Miranda was involved in terrorism and instead spent hours questioning him about the Guardian's reporting on the activities of the US National Security Agency, which has enraged Washington.
"This was obviously designed to send a message of intimidation to those of us working journalistically on reporting on the NSA and its British counterpart, the GCHQ (Government Communications Headquarters)," Greenwald wrote in the Guardian.
"They completely abused their own terrorism law for reasons having nothing whatsoever to do with terrorism."
Miranda, 28, often assists Greenwald with his work, the Guardian said.
He is not an employee of the newspaper but it had paid for his flights.
Miranda said he had been questioned by six agents at Heathrow who confiscated his electronic equipment.
Authorities are under increasing pressure to explain why he had been held.
Britain's opposition Labour party has called for an urgent investigation into whether anti-terror laws had been misused.
Britain's independent reviewer of terror legislation, the barrister David Anderson, says he has asked for a briefing on what he described as an "unusual" case.
