Guardian newspaper editor Alan Rusbridger said the decision to detain David Miranda "seems to me a clear misuse of a law."
Speaking at the Edinburgh International Book Festival, he called for a wider public debate about mass surveillance, praised the value of the information leaked by whistleblower Edward Snowden and described US soldier Bradley Manning's 35-year sentence "staggering".
Mr Rusbridger said the "strong suspicion" has to be that the detention of Mr Miranda at Heathrow Airport was "quite a carefully planned operation and wasn't random".
"There has to be a debate. There hasn't been much of a debate in this country yet because everyone is a bit complacent about it," he said. "In the end it's for people to decide."
An initial examination of material seized from David Miranda at Heathrow Airport "has identified highly sensitive material, the disclosure of which could put lives at risk," Scotland Yard said.
"This [criminal] investigation is at an early stage and we are not prepared to discuss it in any further detail at this stage," a spokesperson added.
The Guardian has welcomed what it called a "partial victory" after David Miranda, who was held at Heathrow Airport under anti-terror laws, was granted a limited injunction by the High Court.
A Guardian News & Media spokesperson said: "We welcome this partial victory but have grave concerns that today's judgment allows police to examine without any legal oversight journalistic material seized from David Miranda.
"It remains our position that David Miranda was involved in legitimate journalistic activity."
The Home Office said it is "right" that the detention of David Miranda be assessed by the independent reviewer of terrorism legislation.
A Home Office spokesman said: "It is right that the independent reviewer of terrorism legislation examines cases like this to ensure that the law has been applied properly."
Earlier today, David Anderson said he had written to the Home Secretary to outline his reasons for looking at Mr Miranda's detention at Heathrow Airport.
The independent reviewer of terrorism legislation has written to the Home Secretary announcing his review of the detention of David Miranda.
David Anderson wrote on Twitter:
The Home Office said it was "pleased" that a High Court ruling allows police to examine material seized from David Miranda at Heathrow:
– Home Office statement
We are pleased the court has agreed that the police can examine the material as part of their criminal investigation insofar as it falls within the purposes of the original Schedule 7 examination and in order to protect national security.
It would be inappropriate for us to comment further on ongoing legal proceedings.
David Miranda's solicitor says he won a "partial victory" in court today by "stopping police in their tracks" in their efforts to use his data.
As a result of Miranda's limited injunction, police will not be able to use his data for their new criminal investigation.
Gwendolen Morgan also told reporters:
- Police have seven days to prove the data poses a genuine threat to national security.
- Miranda's property will be returned at midnight on Saturday.
The Metropolitan police have launched a criminal investigation sparked by "tens of thousands" of pages of data seized from David Miranda at Heathrow Airport, their lawyer Jonathan Laidlaw QC said.
The QC told the court that the police were only "partway through" examining the data.
– Jonathan Laidlaw QC
That which has been inspected contains in the view of the police highly sensitive material, the disclosure of which would be gravely injurious to public safety and thus the police have now initiated a criminal investigation.
I am not proposing to say anything else which may alert potential defendants here or abroad to the nature and the ambit of the criminal investigation which has now begun.
There is an absolutely compelling reason to permit this investigation to continue.
David Miranda, the Guardian journalist's partner who was held at Heathrow Airport under anti-terror laws, has been granted a limited injunction at the High Court.
It prevents the Government and police from "inspecting, copying or sharing" data seized from him during his detention, but still allows examination for national security purposes.