* UK paper has led coverage of material leaked by Snowden
* "We want the stuff back," government agent told paper
* Furore over detention of journalist's partner intensifies (Adds Miranda legal action, new Rusbridger quote, details)
By Estelle Shirbon and Michael Holden
LONDON, Aug 20 (Reuters) - The British authorities forced the Guardian newspaper to destroy material leaked by Edward Snowden, its editor has revealed, calling it a "pointless" move that would not prevent further reporting on U.S. and British surveillance programmes.
In a column on Tuesday, Alan Rusbridger said he had received a call from a government 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) had visited the paper's London offices and watched as computer hard drives containing Snowden material were reduced to mangled bits of 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.
Rusbridger said the "bizarre" episode and the detention at London's Heathrow airport on Sunday of the partner of Guardian journalist Glenn Greenwald showed press freedom was under threat in Britain.
The nine-hour detention under an anti-terrorism law of David Miranda, Greenwald's Brazilian partner, has caused a furore with Brazil, British opposition politicians, human rights lawyers and press freedom watchdogs among those denouncing it.
Greenwald was the first journalist to publish U.S. and British intelligence secrets leaked by Snowden, a former U.S. National Security Agency (NSA) contractor who is wanted in the United States and has found temporary asylum in Russia.
Snowden's leaks have revealed details of NSA and GCHQ surveillance of global communications networks. Washington and London say their security agencies act within the law and the leaks are a threat to national security.
Britain's Home Office, or interior ministry, defended Miranda's detention on Tuesday.
"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 in a statement.
London's Metropolitan Police said Miranda's detention had been "legally and procedurally sound".
Miranda, who was in transit on his way from Berlin to Rio de Janeiro where he lives with Greenwald, was questioned for nine hours before being released without charge, minus his laptop, mobile phone and memory sticks.
He had been ferrying materials obtained from Snowden between Greenwald and Laura Poitras, an independent film-maker based in Berlin who has also published reports based on Snowden material.
Miranda has launched legal action against the British police and government to question the legal basis of his detention and stop the authorities from viewing, copying or passing on his data, his lawyer Gwendolen Morgan told Reuters.
The White House said on Monday Washington was given a "heads up" ahead of Miranda's detention but had not requested it.
Dunja Mijatovic, media freedoms chief at the Organisation for Security and Cooperation in Europe, a 57-nation human rights and security watchdog, said she had written to the British authorities to express concerns about Miranda's detention.
"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 ... It seems that the UK government is using, and quite literally misusing, laws to intimidate journalists and silence its critics," the group said.
Rusbridger said the destruction of the computer material was "pointless" as there were other 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. We just won't do it in London," he said.
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. (Writing by Estelle Shirbon, additional reporting by Andrew Osborn in London and Mark Hosenball in Washington; Editing by Guy Faulconbridge and Peter Graff)