David Miranda - the Brazilian partner of Glenn Greenwald, an American journalist with Britain's Guardian newspaper - was held yesterday as he passed through London's Heathrow Airport on his way home 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" or Government Communications Headquarters, Greenwald wrote in the Guardian.
"They completely abused their own terrorism law for reasons having nothing whatsoever to do with terrorism."
He added: "The last thing it will do is intimidate or deter us in any way from doing our job as journalists."
28-year-old Miranda had been refused access to a lawyer and officials had taken his laptop and mobile phone, Greenwald said.
The British interior ministry did not immediately comment on Miranda's detention, saying it was a police matter.
But authorities were under increasing pressure to explain why he had been held, with Brazil expressing "grave concern" that one of its citizens had been apparently "held incommunicado" at the airport.
Keith Vaz, chairman of the British parliament's home affairs committee, said Miranda's detention was "extraordinary" and he was writing to the police to demand an explanation.
"What is extraordinary is they knew he was the partner (of Greenwald) and therefore it is clear not only people who are directly involved are being sought but also the partners of those involved," he told BBC radio.
Vaz said it was possible the authorities had "a perfectly reasonable explanation", but added: "If we are going to use the (Terrorism) Act in this way, for those issues that are not related to terrorism, then at least we need to know."
London's Metropolitan Police confirmed that a 28-year-old man was detained at Heathrow Airport under anti-terrorism legislation.
"He was not arrested. He was subsequently released," a spokesman said.
"This measure is without justification since it involves an individual against whom there are no charges that can justify the use of that legislation," the ministry said in a statement.
Brazil expects there to be no repeat of the incident, it warned.
Greenwald, a well-known journalist in the US, analysed and published information on documents released by former US security contractor Snowden revealing huge electronic surveillance operations by the NSA.
Snowden has been granted asylum in Russia after spending five weeks in limbo at a Moscow airport attempting to avoid extradition to the US. He is wanted by Washington on espionage charges.
The Guardian said it was "dismayed" by Miranda's detention and was seeking "clarification" from the British authorities.
Greenwald said Miranda was on his way to the couple's home in Rio de Janeiro when he was detained. He had stayed in Berlin with Laura Poitras, a US filmmaker who worked on the NSA stories.
Greenwald said he had received a phone call regarding Miranda from someone identifying himself as a security official at Heathrow.
"The security official told me that they had the right to detain him for up to nine hours in order to question him, at which point they could either arrest and charge him or ask a court to extend the question time," he wrote.
"The official - who refused to give his name but would only identify himself by his number: 203654 - said David was not allowed to have a lawyer present, nor would they allow me to talk to him."
Amnesty International said Miranda was "clearly a victim of unwarranted revenge tactics".
Miranda's detention sparked a furore on Twitter, with British actor and campaigner Stephen Fry writing: "Holy CRAP - what is our government up to?"
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