The Home Office has defended the decision to detain the partner of the Guardian journalist behind a series of stories that exposed the activities of British and American intelligence agencies.
David Miranda, the partner Glenn Greenwald, was held by police at Heathrow Airport for nine hours on Sunday under anti-terror laws.
Greenwald - the reporter who interviewed American whistle-blower Edward Snowden - called Miranda's detention a "profound attack on press freedoms and the news-gathering process".
"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 (US National Security Agency) and GCHQ," he said.
On Tuesday morning the Home Office defended the decision. A spokesman said: "The government and the 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.
"Those who oppose this sort of action need to think about what they are condoning. This is an ongoing police inquiry so we will not comment on the specifics."
Scotland Yard has said the detention was "legally and procedurally sound". Miranda said he was questioned by six agents on his "entire life" while being held at Heathrow for nine hours - the legal limit before a suspect must be charged or released.
Miranda was stopped at 8.30am on Sunday when returning from a trip to Berlin. 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.
On Tuesday the editor of The Guardian, Alan Rusbridger, revealed British intelligence officials smashed hard drives in the basement of the newspaper's London headquarters.
He claimed agents threatened the paper with legal action over the Edward Snowden intelligence leaks and told him: "You've had your fun, now we want the stuff back."
White House spokesman Josh Earnest said the British government gave the United States advance notice that London police intended to detain Mr Miranda but added that the US did not request the detention and was not involved in the decision.