British authorities detained the Brazilian partner of the Guardian journalist Glenn Greenwald, who published the revelations of mass surveillance programmes by the US National Security Agency, for nine hours under the Terrorism Act and confiscated his possessions.
David Michael Miranda, who is in a relationship with Greenwald, the first to interview American whistleblower Edward Snowden, was stopped at 8.30am at London's Heathrow Airport returning from a trip to Berlin to his home in Rio de Janeiro.
He was questioned under schedule 7 of the Terrorism Act 2000 which applies only at airports, ports and border areas, allowing officers to stop, search, question and detain individuals, the Guardian reported.
The 28-year-old was held for nine hours - the maximum the law allows before officers must release or formally arrest the individual - before being released without charge.
But the newspaper reported his electronic possessions including his mobile phone, laptop, camera, memory sticks, DVDs and games consoles were confiscated.
The police were severely criticised by the Brazilian government, who issued a statement saying: "This measure is without justification since it involves an individual against whom there are no charges that can legitimate the use of that legislation.
"The Brazilian government expects that incidents such as the one that happened to the Brazilian citizen today are not repeated."
“What’s amazing is this law, called the Terrorism Act, gives them a right to detain and question you about your activities with a terrorist organization or your possible involvement in or knowledge of a terrorism plot,” Greenwald told the New York Times.
“The only thing they were interested in was NSA documents and what I was doing with Laura Poitras. It’s a total abuse of the law. This is obviously a serious, radical escalation of what they are doing. He is my partner. He is not even a journalist.”
While in Berlin, Miranda visited Laura Poitras, the US film-maker who has also been working on the Snowden files with Greenwald and the Guardian.
Ironically, Poitras, whose work usually involves sensitive national security issues, re-located to Berlin from America because she had grown tired of being constantly detained and questioned at airports.
"This is a profound attack on press freedoms and the news gathering process," said Greenwald.
"To detain my partner for a full nine hours while denying him a lawyer, and then seize large amounts of his possessions, is clearly intended to send a message of intimidation to those of us who have been reporting on the NSA and GCHQ.
"The actions of the UK pose a serious threat to journalists everywhere.
"But the last thing it will do is intimidate or deter us in any way from doing our job as journalists.
"Quite the contrary: it will only embolden us more to continue to report aggressively."
According to official figures, more than 97% of examinations under schedule 7 last less than an hour, and only one in 2,000 people detained is kept for more than six hours.
Since June 5, Greenwald has written a series of stories revealing the NSA's electronic surveillance programmes.
The broadsheet also published stories about blanket electronic surveillance by Britain's GCHQ, also based on documents from Mr Snowden.
A Guardian spokesman said: "We were dismayed that the partner of a Guardian journalist who has been writing about the security services was detained for nearly nine hours while passing through Heathrow airport.
"We are urgently seeking clarification from the British authorities."
Widney Brown, Amnesty International's senior director of international law and policy, said: "It is utterly improbable that David Michael Miranda, a Brazilian national transiting through London, was detained at random, given the role his partner has played in revealing the truth about the unlawful nature of NSA surveillance.
"David's detention was unlawful and inexcusable. He was detained under a law that violates any principle of fairness and his detention shows how the law can be abused for petty vindictive reasons.
"There is simply no basis for believing that David Michael Miranda presents any threat whatsoever to the UK Government.
"The only possible intent behind this detention was to harass him and his partner, Guardian journalist Glenn Greenwald, for his role in analysing the data released by Edward Snowden."
She added: "States cannot pass anti-terror acts and claim they are necessary to protect people from harm and then use them to retaliate against someone exercising his rights.
"By targeting Miranda and Greenwald, the UK authorities are also sending a message to other journalists that if they maintain their independence and report critically about governments, they too may be targeted."
A Metropolitan Police spokesman said: "At 08:05 on Sunday 18 August 2013 a 28-year-old man was detained at Heathrow Airport under Schedule 7 of the Terrorism Act 2000.
"He was not arrested. He was subsequently released at 17:00."