Pressure is mounting on police to justify the detention of a journalist's partner under terror laws.
Senior politicians and an independent reviewer have said police must explain why David Miranda was detained for nine hours at Heathrow Airport.
Mr Miranda's partner is a journalist who published documents leaked by US whistleblower Edward Snowden.
Police have not said why Mr Miranda was held, but he said he was kept in a room and quizzed by "six agents".
Keith Vaz, chairman of the Home Affairs Select Committee, and shadow home secretary Yvette Cooper said police must explain why terrorism powers were used.
Brazil's foreign minister Antonio Patriota said he would call his UK counterpart William Hague to tell him the detention of Mr Miranda - a Brazilian national - was "not justifiable" and ask him to ensure it "won't happen again".
Questions 'about everything'
Mr Miranda, 28, was held at Heathrow on Sunday on his way from Berlin to Rio de Janeiro, where he lives with his partner, Guardian journalist Glenn Greenwald.
"I remained in a room, there were six different agents coming and going, talking to me," Mr Miranda said.
"They asked questions about my entire life, about everything.
"They took my computer, video game, mobile phone, my memory cards, everything."
In Germany, Mr Miranda had been staying with US film-maker Laura Poitras, who has also been working on the Snowden files with Mr Greenwald and the Guardian, according to the newspaper.
His flights were being paid for by the Guardian. A spokesman said he was not an employee of the newspaper but "often assists" with Mr Greenwald's work.
Mr Miranda was detained under schedule 7 of the Terrorism Act 2000. This allows police to hold someone at an airport for up to nine hours for questioning about whether they have been involved with acts of terrorism.
Anyone detained must "give the examining officer any information in his possession which the officer requests". Any property seized must be returned after seven days.
The Independent Reviewer of Terrorism Legislation, David Anderson QC, said it was very unusual for someone to be held for the full nine hours, and he wanted to "get to the bottom" of what had happened.
He said he had asked the Home Office and Scotland Yard for a full briefing.
The Guardian said it was "dismayed" by the detention and was "urgently seeking clarification from the British authorities" as to why it had happened.
The Metropolitan Police confirmed a 28-year-old man was held from 08:05 BST until 17:00 BST on Sunday under schedule 7 and was not arrested.
According to the Home Office, more than 97% of examinations under schedule 7 last less than an hour.
Mr Greenwald said the British authorities' actions in holding Mr Miranda amounted to "bullying" and linked it to his writing about Mr Snowden's revelations concerning the US National Security Agency (NSA).
He said it was "clearly intended to send a message of intimidation to those of us who have been reporting on the NSA and [UK intelligence agency] GCHQ".
He told the BBC police did not ask Mr Miranda "a single question" about terrorism but instead asked about what "Guardian journalists were doing on the NSA stories".
Mr Greenwald said he would respond by writing reports "much more aggressively than before".
"I have lots of documents about the way the secret services operate in England," he said.
"I think they are going to regret what they did."
Mr Snowden, who has been granted temporary asylum in Russia, leaked details of extensive internet and phone surveillance by American intelligence services.
According to the Guardian, he passed "thousands of files" to Mr Greenwald, who has written a series of stories about surveillance by US and UK authorities.
Mr Vaz said police must "of course" question people if they have "concerns" about what they are doing in the UK.
"What is extraordinary is they knew he was the partner [of Mr Greenwald] and therefore it is clear not only people who are directly involved are being sought but also the partners of those involved," he said.
"Bearing in mind it is a new use of terrorism legislation to detain someone in these circumstances... I will write to the police to ask for the justification of the use of terrorism legislation - they may have a perfectly reasonable explanation."
Ms Cooper said the situation must be "investigated and clarified urgently", adding: "The public support for these powers must not be endangered by a perception of misuse."
In a statement, the Liberal Democrats said police should use schedule 7 powers "proportionately and for good reason".
Journalism 'not terrorism'
Dr David Lowe, a former counter-terrorist detective, said the length of the detention might be explained by the "volume of documentation" carried by Mr Miranda.
He said the amount of information revealed by Mr Snowden to the Guardian was not yet known, but police might have kept Mr Miranda for the full nine hours allowed because they had lots of data to go through.
Dr Lowe also said Mr Miranda might have been targeted because of the "top secret" information police thought he was carrying, rather than because of his relationship with Mr Greenwald.
But journalists' groups have accused authorities of misusing terrorism laws.
Bob Satchwell, of the Society of Editors, said the incident was "another example of the dangerous tendency" for authorities to "assume that journalists are bad when in fact they play an important part in any democracy."
He added: "Journalism may be embarrassing and annoying for governments but it is not terrorism."