Britain's authorities said today the use of anti-terrorism powers to detain the partner of a journalist who wrote about US and British surveillance programmes based on leaks by Edward Snowden was "legally and procedurally sound".

David Miranda, partner of U.S. journalist Glenn Greenwald, was questioned for nine hours on Sunday at London's Heathrow Airport before being released without charge, prompting calls for an explanation of why anti-terrorism powers were used to detain the Brazilian citizen.

The Metropolitan Police Service (MPS) said in a statement the examination of the 28-year-old man was "necessary and proportionate" and he had been offered legal representation and was attended by a solicitor.

"No complaint has been received by the MPS at this time," the statement said.

Miranda was detained under Schedule 7 of the Terrorism Act 2000, which allows police to stop and question people travelling through ports and airports to determine whether they are involved in planning terrorist acts.

The MPS said there were safeguards in place to ensure that the Schedule 7 power was used "appropriately and proportionately".

The Guardian, which has published articles written by Greenwald, said yesterday it was "dismayed" at Miranda's detention and that it would be pressing British authorities for clarification.

Separately, the newspaper said the government had threatened legal action against it unless it destroyed the classified documents it got from Snowden or handed them back.

The opposition Labour Party also urged the authorities to explain how they could justify using the Schedule 7 measure in the case of Miranda, and rights activists accused the authorities of harassing anyone connected to Snowden.