* Row over detention of journalist's partner intensifies
* Government insists police acted to prevent terrorism
* Guardian complains was forced to destroy computers
* "We want the stuff back," government agent told paper (Recasts with new headline, Home Secretary comment, detail)
By Estelle Shirbon and Michael Holden
LONDON, Aug 20 (Reuters) - The British government, accused of abusing media freedom, said on Tuesday police were right to detain a journalist's partner if they thought lives might be at risk from data he was carrying from fugitive U.S. intelligence officer Edward Snowden.
Facing legal and diplomatic complaints after police held Guardian writer Glenn Greenwald's Brazilian partner for nine hours on Sunday - and accused by the newspaper of forcing it to trash computers holding copies of Snowden's data - the interior minister said officers were entitled to take security measures.
Home Secretary Theresa May said police held David Miranda at a London airport under anti-terrorism powers, which allow for action to prevent stolen data aid terrorists. Material from Snowden, published by the Guardian, has revealed extensive U.S. and British surveillance of global communications networks.
"It's absolutely right that if the police believe that somebody is in possession of highly sensitive, stolen information, that could help terrorists, that could risk lives, lead to a potential loss of life, the police are able to act and that's what the law enables them to do," May told the BBC.
As interior minister, May said she was briefed in advance that Miranda might be stopped but she stressed that she did not decide whom the police detained. The United States - which has charged Snowden, now in Russia, with spying - said Britain gave it a "heads up" but it did not ask for Miranda to be questioned.
A British lawyer who launched an action on the Brazilian's behalf to question the legal basis of his detention said police seized a laptop computer, a telephone, memory sticks, a computer hard drive and a games console from him. He was released without charge after reaching a time limit on such detentions.
Miranda had been in transit at Heathrow, carrying material from Snowden that was being passed from Berlin-based American documentary film-maker Laura Poitras to Greenwald, an American writer for Britain's Guardian who lives in Rio de Janeiro.
"These items contain sensitive, confidential journalistic material and should not have been seized," Miranda's London lawyers wrote in a letter to May and the police.
The Brazilian government has complained to Britain. The action against Miranda was also denounced by British opposition politicians, human rights lawyers and press freedom watchdogs.
The leaks by Snowden, a former U.S. National Security Agency (NSA) contractor who has found temporary asylum in Russia, gave details of NSA and British surveillance of public telephone and Internet traffic. Washington and London say their agencies act within their laws and the leaks threaten their security.
"YOU'VE HAD YOUR FUN"
On Tuesday, Guardian editor Alan Rusbridger said he received a call from a British official a month ago who told him: "You've had your fun. Now we want the stuff back." The paper had been threatened with legal action if it did not comply.
Later, two "security experts" from the secretive Government Communications Headquarters (GCHQ) visited the paper's London head office and watched as computer hard drives containing copies of Snowden material were reduced to mangled metal.
Asked by the BBC who he thought was behind those events, Rusbridger said he had "got the sense there was an active conversation" involving government departments, intelligence agencies and the prime minister's Downing Street office.
Downing Street and GCHQ declined to comment.
Dunja Mijatovic, media freedoms chief at the 57-nation Organisation for Security and Cooperation in Europe, said she had written to Britain to express concerns: "The detention can be interpreted as putting pressure on Glenn Greenwald after his recent reporting on security issues in the Guardian," she wrote.
Britain also came under attack from press freedom group Index on Censorship, which denounced the forced destruction of computers revealed by Rusbridger in his Tuesday column.
"It is clear that the Snowden and NSA story is strongly in the public interest," the group said. "It seems that the UK government is using, and quite literally misusing, laws to intimidate journalists and silence its critics."
Rusbridger said the destruction of the computer material was "pointless" as there were copies of what was lost, and it would not stop the Guardian from pursuing Snowden stories: "We will continue to do patient, painstaking reporting on the Snowden documents," he said. "We just won't do it in London."
A British source with knowledge of the security services said GCHQ had no powers to seize material from the Guardian, but could have accused the paper of possessing stolen materials and demanded they be destroyed. (Additional reporting by Andrew Osborn and Belinda Goldsmith in London and Mark Hosenball in Washington; Editing by Guy Faulconbridge, Peter Graff and Alastair Macdonald)