Home Secretary Theresa May has defended the police's use of anti-terror laws to hold and question David Miranda, the partner of a Guardian journalist.
She said it "was right" if police thought that Mr Miranda was holding information useful to terrorists.
But former Lord Chancellor Lord Falconer said it was not what the powers were meant for.
Mr Miranda is the partner of Glenn Greenwald, who has been reporting on US and British surveillance programmes.
He was held for nine hours at Heathrow Airport on Sunday and said his interrogators threatened he could go to prison if he did not co-operate.
Brazilian national Mr Miranda, 28, was detained under schedule 7 of the Terrorism Act 2000 as he travelled from Berlin to Rio de Janeiro, where he lives with Mr Greenwald.
The law allows police to hold someone for up to nine hours for questioning about whether they have been involved with acts of terrorism.
Mrs May told BBC Radio 4's World At One programme: "If the police believe someone has in their possession highly sensitive stolen information that could help terrorists that could lead to a loss of life, it is right the police should act.
BBC News - David Miranda detention: Police right to act, says May
I hope the government realise that they have only damaged themselves in this one. Mrs May seems to have the habit of attempting to put the boot in and it always lands in her own mouth.
She may want a drink and some laxatives with that one by the time it's over. Worst thing is if he sues and wins it will be the taxpayer who ends up paying for the governments mistake.
Even the Russsians are taking the p**s now:
How long before China pipes up and asks whether the UK is a secretive and not so free state which abuses the rights of individuals?
She said it "was right" if police thought that Mr Miranda was holding information useful to terrorists.
But former Lord Chancellor Lord Falconer said it was not what the powers were meant for.
Mr Miranda is the partner of Glenn Greenwald, who has been reporting on US and British surveillance programmes.
He was held for nine hours at Heathrow Airport on Sunday and said his interrogators threatened he could go to prison if he did not co-operate.
Brazilian national Mr Miranda, 28, was detained under schedule 7 of the Terrorism Act 2000 as he travelled from Berlin to Rio de Janeiro, where he lives with Mr Greenwald.
The law allows police to hold someone for up to nine hours for questioning about whether they have been involved with acts of terrorism.
Mrs May told BBC Radio 4's World At One programme: "If the police believe someone has in their possession highly sensitive stolen information that could help terrorists that could lead to a loss of life, it is right the police should act.
BBC News - David Miranda detention: Police right to act, says May
I hope the government realise that they have only damaged themselves in this one. Mrs May seems to have the habit of attempting to put the boot in and it always lands in her own mouth.
But Lord Falconer, who helped to bring in the anti-terror legislation, said: "It does look like the wrong powers were used. Schedule 7 of the terrorism act is to be used... to discover whether somebody is a terrorist."If you know the person is not a terrorist, that power is not open to you." He said the justification given by the home secretary "doesn't look right". And Lord Carlisle, the government's former terror watchdog, said whoever made the decision to use the law must be held to account.
Even the Russsians are taking the p**s now:
Russian Foreign Ministry spokesman Alexander Lukashevich was also critical. "The measures taken by the British authorities towards the Guardian newspaper are out of tune with the British side's statements on commitments to universal standards of human rights, including the area of mass media, protecting the rights of journalists, and private life," he said.
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