U.K. authorities didn't confirm they conduct that type of mass spying on residents. But they said collecting the information would be justified under broad warrants that don't require naming a specific person or location as a target.

The 48-page testimony came in response to a legal challenge brought last year by several privacy-advocacy organizations, which said the mass spying described by former U.S. National Security Agency contractor Edward Snowden was illegal.

The disclosure is the latest to provide some insight into how the U.K. government balances personal privacy against security forces' efforts to address criminal and terrorist threats and enterprises using new communications tools in the Internet age.

In the documents, released on Tuesday by Privacy International, a London-based group that filed the initial complaint in July 2013, Charles Farr, director general of the Office for Security and Counter Terrorism in the U.K. Home Office, delineated between two types of communication, with different standards for surveillance.

Emails between two British residents are considered "internal" communications, even if they passed through a server in the U.S. or elsewhere, and require a warrant that names a person or place as a target, similar to a phone call between two British residents inside the U.K. Google searches, however, would be considered "external" communications because they would constitute communication between U.K. residents and Google servers, likely in a foreign country, Mr. Farr said. Those external searches also require a warrant, but one that doesn't have to name a specific person or place.

Mr. Farr also said the government could inadvertently gather emails between two U.K. residents while sweeping up foreign emails.

Privacy groups said they were concerned with the way the government viewed the Internet. "What we search for is far more sensitive than the list of who you actually call," said Gus Hosein, executive director of Privacy International. "They can get insight into you as a human being, as opposed to what you choose to share with people."

Michael Bochenek, a lawyer with Amnesty International, another group joining the complaint, said the type of information gathering described by the U.K. Home Office shouldn't be seen as any different than a person's rights to privacy while looking up library books in a public card catalog. "This case has really brought home to us how much power these agencies have to decide the parameters of their own conduct," he said.

A Home Office spokeswoman declined further comment on the testimony. A hearing in front of the Investigatory Powers Tribunal, a British body that looks into complaints about surveillance, is expected to begin on July 14.