London - British authorities faced increasing pressure on Monday to explain why they used anti-terror laws to detain the partner of a journalist who worked with US intelligence leaker Edward Snowden.
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, aged 28, often assists Greenwald with his work, the Guardian said.
He is not an employee of the newspaper but it paid for his flights. He stayed in Berlin with Laura Poitras, a US filmmaker who has been working with the Guardian.
Miranda said he had been questioned by six agents at Heathrow who confiscated his electronic equipment.
"The minute I stepped out of the plane they took me away to a small room with four chairs and a machine for taking fingerprints," he told the Guardian.
"They asked questions about my entire life, about everything. They took my computer, video game, mobile phone, my memory card."
A spokesperson for Prime Minister David Cameron said the British government takes "all necessary steps to protect the public from individuals who pose a threat to national security".
"But it is for the police to decide when it is necessary and proportionate to use these powers," the spokesperson said.
London's Metropolitan Police on Monday insisted their actions were "reviewed throughout to ensure the examination was both necessary and proportionate" and that its use of anti-terror powers was "legally and procedurally sound".
The force also dismissed reports that Miranda was denied legal representation and said that a solicitor was present.
The United States denied it asked Britain to detain Miranda.
"There was a heads-up that was provided by the British government, so this is something we had an indication was likely to occur," White House deputy spokesperson Josh Earnest told reporters.
"But it's not something that we requested, and it's something that was done specifically by the British law enforcement officials."
But authorities were under increasing pressure to explain why Miranda was held, with Brazil expressing "grave concern" that one of its citizens had apparently been "held incommunicado".
Brazil's foreign ministry said its embassy in London had contacted British officials prior to Miranda's release and that Brazil would also be seeking an explanation.
"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.
Arriving to meet Miranda at Rio's airport, meanwhile, Greenwald said he was now even more determined to continue reporting on the intelligence leaks - with a new focus on Britain.
"Now I will be more radical in my reporting," he warned. "This was a clear attempt at intimidation."
Rights group Amnesty International said Miranda was "clearly a victim of unwarranted revenge tactics", while Reporters Without Borders said it was "outraged" by his detention.
Guardian editor Alan Rusbridger on Tuesday claimed the British government forced the paper to destroy files or face a court battle over its publication of Snowden's revelations.
Rusbridger says he was contacted by "a very senior government official claiming to represent the views of the prime minister" which led to two meetings in which "he demanded the return or destruction of all the material we were working on."
He claimed authorities told him: "You've had your fun. Now we want the stuff back."
Snowden was granted asylum in Russia after spending five weeks in limbo at a Moscow airport attempting to avoid extradition to the United States, where he is wanted on espionage charges.