The partner of the journalist who first reported on the mass surveillance by the U.S. National Security Agency was detained at London's Heathrow Airport on Sunday and questioned for hours under Britain's anti-terrorism law, according to the Guardian newspaper.

American writer Glenn Greenwald, who broke revelations by whistleblower Edward Snowden in a series of articles for the London-based Guardian, called the treatment of his partner, David Miranda, "despotic."

"This is obviously a rather profound escalation of their attacks on the news-gathering process and journalism," Greenwald wrote in a blog post. "It's bad enough to prosecute and imprison sources. It's worse still to imprison journalists who report the truth.

"But to start detaining the family members and loved ones of journalists is simply despotic. Even the Mafia had ethical rules against targeting the family members of people they felt threatened by."

Miranda was travelling from Berlin to his and Greenwald's home in Rio de Janeiro via London when he was detained at Heathrow airport for nearly nine hours, searched and questioned under Britain's Terrorism Act of 2000.

He was eventually let go, the Guardian said, but only after authorities seized his cellphone, laptop, camera, some DVDs and even a gaming console.

Former CIA employee Edward Snowden, who leaked top-secret files about sweeping U.S. surveillance programs to a Guardian journalist and an American filmmaker, is shown on a TV screen at a shopping mall in Hong Kong. (Vincent Yu/Associated Press)

Miranda had been visiting another journalist, award-winning and Oscar-nominated documentary filmmaker Laura Poitras, in Berlin.

Poitras — an American who, according to reports, has herself been repeatedly detained, searched and interrogated for hours by authorities on return trips to the U.S. — was the first contact point for Snowden last January when he sought to leak top-secret files about the NSA's extensive domestic spying. She collaborated with Greenwald on his NSA stories for the Guardian, and shared several bylines with him.

London police confirmed that "a 28-year-old man was detained at Heathrow airport under schedule 7 of the Terrorism Act 2000," but would not say why, the Guardian reported.

"They obviously had zero suspicion that David was associated with a terrorist organization or involved in any terrorist plot. Instead, they spent their time interrogating him about the NSA reporting which Laura Poitras, the Guardian and I are doing," Greenwald said.

"They completely abused their own terrorism law for reasons having nothing whatsoever to do with terrorism, a potent reminder of how often governments lie when they claim that they need powers to stop 'the terrorists.' "

Leaks reveal U.K. involvement

Greenwald wrote his first story in early June based on files provided by Snowden. It disclosed that the NSA had authority under a secret court order to collect, on an "ongoing, daily basis," the records of all landline and mobile telephone calls of Verizon customers, both within the U.S. and between the U.S. and other countries.

Intelligence experts said at the time that there was every reason to believe similar orders were in place for other U.S. phone companies.

The following day, the Guardian and the Washington Post separately exposed a joint FBI-NSA program called Prism that uses secret access to data from the biggest Internet companies to eavesdrop on people's emails, instant messages, chats, documents and connection logs. The companies include Microsoft, Yahoo, Google, Facebook, PalTalk, AOL, Skype, YouTube and Apple.

Subsequent stories have revealed that though these programs are only supposed to track communications where at least one party is outside the U.S., the NSA has in fact broken privacy rules and overstepped its legal authority thousands of times by conducting surveillance on Americans or foreign intelligence targets inside the United States.

There have been disclosures about the British intelligence apparatus, as well. A Guardian article based on Snowden's documents said the British eavesdropping agency GCHQ repeatedly hacked into foreign diplomats' phones and emails when the U.K. hosted international conferences, even going so far as to set up a bugged Internet café in an effort to get an edge in high-stakes negotiations.

The Guardian also reported that the GCHQ has tapped fibre-optic cables that carry international phone and internet traffic and is sharing vast quantities of personal information with the NSA.