(Bloomberg) -- Lawyers for U.K. spy agencies said at the start of a trial over privacy concerns that the bulk interception of communications is critical in protecting the country against terrorism.

Without the ability to collect mass data, MI5, MI6 and GCHQ "would be less effective in protecting the U.K. against threats such as terrorism, cyber threats or espionage," government lawyers said in court documents responding to a suit from Privacy International, an advocacy group that’s seeking to clarify whether the U.K. government has been acting lawfully.

"Exploitation of Bulk Personal Datasets is an essential tool that is used on a daily basis, in combination with other capabilities, right across the intelligence services’ operations," an unidentified GCHQ witness said in documents prepared for the trial at a special London court.

Western governments have had to tread a fine line between civil liberties and protecting civilians in the face of an increase in extremist violence across Europe and the U.S. In the past week there have been attacks in Germany and France that have seen dozens of people killed or injured.

Foiled Attacks

The U.K. faces the threat of a domestic attack not just from supporters of Islamist extremism, but also Northern Ireland and authoritarian regimes, government lawyers said. U.K. agencies foiled six attacks in the country in the 12 months before September 2015, they said.

But lawyers for Privacy International said there weren’t adequate measures in place to ensure spy agencies don’t overstep the mark when it comes to bulk interception.

"There are no sufficient safeguards to challenge the aggressive and expansive interpretation of the security services legal powers," Thomas De La Mare, Privacy International’s lawyer, said in court Tuesday.

Current bulk-collections include location and call data for everyone’s mobile phone in the U.K. for one year, Privacy International said in its prepared arguments for the Investigatory Powers Tribunal, a special court that investigates complaints about spying.

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