The partner of journalist Glenn Greenwald, who brought leaks from former U.S. spy agency contractor Edward Snowden to world attention, went to court on Wednesday to challenge the legality of his detention by British police in August.
David Miranda was held under anti-terrorism legislation and questioned for nine hours when he landed at London's Heathrow Airport en route from Berlin to Rio De Janeiro, Brazil.
After his return to Rio, Miranda filed a suit seeking the return of materials seized from him by British authorities and a High Court review of the basis for his detention.
Snowden's revelations about the extent of electronic surveillance by the United States and its allies, sometimes of each other, have embarrassed those countries but also, according to the governments, given valuable information to enemies.
British authorities have said that items seized from Miranda included electronic media containing 58,000 documents from the U.S. National Security Agency, Snowden's former employer, and from its British counterpart, Government Communications Headquarters (GCHQ).
Arguing that Britain had misused anti-terrorism laws to detain Miranda, he and Greenwald said in statements submitted to the High Court that Britain wrongly equated journalism with terrorism and improperly confiscated journalistic material.
"Responsible publication of sensitive material is not an act of terrorism. And those involved in such work - the journalists and those who assist them - are not involved in terrorist activity," Greenwald said in his statement.
"It is irrational to suggest that sensitive material in the hands of responsible journalists and their staff should be equated with sensitive material in the hands of terrorists," he added.
"Such suggestions reveal a disturbing misunderstanding of the work that journalists do."
Miranda said he believed the British authorities had long planned to stop him, with the aim of viewing the material obtained from Snowden.
"They had always intended to detain me for some time, because their primary purpose was to access material I was carrying rather than to investigate or question me about terrorism," Miranda said in his statement.
Miranda's lawyers who failed to stop the government examining the data seized after Miranda's detention.
Britain said at the time that Snowden's leaks, published by Britain's Guardian newspaper, the Washington Post and others, had damaged national security and endangered lives.
Deputy National Security Adviser Oliver Robbins argued in evidence submitted on Wednesday that British security services had been obliged to ascertain whether the data being ferried by Miranda was a threat to Britain's security.
"When it became apparent that Mr Miranda would be returning to Brazil via Heathrow airport, the Security Service had a national security duty to establish the nature of Mr Miranda's activity, to gain an insight into what, if any, UK, classified material Mr Miranda was carrying," he said in a statement.
The hearing was expected to last two days.