Attending a vigil in Trafalgar Square in solidarity with a unity march in Paris after 17 people were killed by terrorists in the French capital, Boris Johnson said:
I'm not interested in this civil liberties stuff. If they're a threat, I want their emails and calls listened to.
The mayor of London's remarks came as prime minister David Cameron admitted there were "things to learn" from violence witnessed in Paris and that the Islamist terror threat would be with us "for many years to come".
He is due to hold talks with UK security chiefs on Monday to discuss what steps, if any, should be taken to grant new powers to GCHQ.
Labour leader Ed Miliband, who like Mr Cameron attended the unity rally in Paris, instead called for a "cautious and considered" response.
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