LONDON (CNN) — David Miranda, the partner of Guardian journalist Glenn Greenwald, is taking legal action against the British government after he was detained and searched at Heathrow airport over the weekend, his lawyers said Tuesday.
Miranda argues it was illegal for police to seize data from him and wants to ensure that they do not do anything with the material until a judge has heard his claim.
"We are most concerned about the unlawful way in which these powers were used and the chilling effect this will have on freedom of expression," said Kate Goold, of the law firm Bindmans, which is representing Miranda.
Miranda, 28, was held for nearly nine hours Sunday while reportedly on his way home to Brazil after leaving Berlin. Authorities seized his laptop, phone, USB sticks and other materials, The Guardian reported.
Miranda's partner, Greenwald, broke the story about the existence of a U.S. National Security Agency program that is thought to have collected large amounts of phone and Internet data. Information for the story came from Edward Snowden, a former contractor for the NSA.
Greenwald told Brazilian media Monday that UK authorities are "are going to regret what they did" to his partner.
But Britain's Home Office Tuesday defended Miranda's detention, saying the government and police "have a duty to protect the public and our national security."
"If the police believe that an individual is in possession of highly sensitive stolen information that would help terrorism, then they should act and the law provides them with a framework to do that," it said.
"Those who oppose this sort of action need to think about what they are condoning."
London's Metropolitan Police, which takes a lead role in counterterrorism efforts in Britain, did not name Miranda in its statement but said its actions were "legally and procedurally sound."
"The examination of a 28-year-old man under Schedule 7 of the Terrorism Act 2000 at Heathrow Airport on Sunday 18 August was subject to a detailed decision making process," the statement said.
"The procedure was reviewed throughout to ensure the examination was both necessary and proportionate."
The statement describes the law under which Miranda was detained as "a key part of our national security capability which is used regularly and carefully by the Metropolitan Police Service to help keep the public safe."
Safeguards are in place to ensure that the power provided under Schedule 7 of the Terrorism Act 2000 is "used appropriately and proportionately," it added.
'No protection for journalists'
But The Guardian's editor, Alan Rusbridger, gave a different assessment of the legislation in an editorial published by the newspaper late Monday.
"Under this measure -- uniquely crafted for ports and airport transit areas -- there are none of the checks and balances that apply once someone is in Britain proper," he wrote. "There is no need to arrest or charge anyone and there is no protection for journalists or their material. A transit lounge in Heathrow is a dangerous place to be."
In his editorial, Rusbridger went on to detail various meetings he says have taken place in the past few weeks with UK government officials and "shadowy Whitehall figures" in connection with Greenwald's reporting.
"The demand was the same: hand the Snowden material back or destroy it," he said. Otherwise, the government would pursue legal action to force its surrender, he says he was told.
Despite his protestations, Rusbridger said, the authorities were unmoved -- so two security experts from GCHQ, the British equivalent of the NSA, then oversaw "the destruction of hard drives in the Guardian's basement just to make sure there was nothing in the mangled bits of metal which could possibly be of any interest to passing Chinese agents."
But Rusbridger points out that The Guardian's analysis of the information leaked by Snowden to Greenwald will continue, just not in Britain.
Rusbridger's allegations have worried campaigners for freedom of information.
"Using the threat of legal action to force a newspaper into destroying material is a direct attack on press freedom in the UK," Kirsty Hughes, chief executive of Index on Censorship, said Tuesday.
"It is unclear which laws would have been used to force The Guardian to hand over its material but it is clear that the Snowden and NSA story is strongly in the public interest.
"Coming on the back of the detention of David Miranda, it seems that the UK government is using, and quite likely misusing, laws to intimidate journalists and silence its critics."
Agents asked 'about my entire life'
Greenwald told Brazil's Globo TV on Monday in Rio de Janeiro that he would write his stories "a lot more aggressively now," following his partner's detention.
"I am going to publish many more documents now. I am going to publish a lot about England, too, I have a lot of documents about the espionage system in England. Now my focus is going to be that as well."
Miranda, also speaking to Globo TV in Rio, said agents were asking him questions "about my entire life."
"I was in a room, there were six different agents coming in and out and talking to me," he said. "They took my computer, video games, cell phone, everything."
"This is obviously a rather profound escalation of their attacks on the news-gathering process and journalism," Greenwald wrote for The Guardian.
"It's bad enough to prosecute and imprison sources. It's worse still to imprison journalists who report the truth. But to start detaining the family members and loved ones of journalists is simply despotic. Even the Mafia had ethical rules against targeting the family members of people they felt threatened by."
The White House was given a "heads up" by the UK government about the move, spokesman Josh Earnest said Monday.
So the United States knew it "was likely to occur, but it's not something that we've requested and it's something that was done specifically by the British law enforcement officials there," he said.
He would not comment on whether the United States has obtained material from Miranda's laptop -- and would not say whether President Barack Obama condemns the detention.
Brazil's Foreign Ministry issued a statement Sunday expressing "grave concern" about the incident. Anger erupted in Brazil when citizens learned of NSA spying on Brazil.
"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 ministry said.
According to The Guardian, nine hours is the maximum time allowed before authorities must either release or arrest a detained individual.
While in Berlin, Miranda stayed with filmmaker Laura Poitras, who has worked "extensively" with Greenwald on his stories about the NSA, the reporter wrote.
The Guardian reported that it paid for Miranda's flights. "Miranda is not a Guardian employee but often assists Greenwald in his work," the newspaper said.
Rusbridger, in his editorial, wrote that "Miranda is not a journalist, but he still plays a valuable role in helping his partner do his journalistic work."
He added that Greenwald's work on the reams of material provided by Snowden has been "immensely complicated by the certainty that it would be highly unadvisable for Greenwald (or any other journalist) to regard any electronic means of communication as safe. The Guardian's work on the Snowden story has involved many individuals taking a huge number of flights in order to have face-to-face meetings."
Miranda is quoted by The Guardian as saying: "So they think I have a big connection. But I don't have a role. I don't look at documents. I don't even know if it was documents that I was carrying. It could have been for the movie that Laura is working on."
-- CNN's Bryony Jones and Caroline Paterson contributed to this report.