Media playback is unsupported on your device Media caption Theresa May: "The terrorist threats to this country and our interests are changing faster than at any time since 9/11"

"There is no programme of mass surveillance and there is no surveillance state," Home Secretary Theresa May has said.

Speaking at the Lord Mayor's Defence and Security Lecture at Mansion House, in the City of London, Mrs May dismissed recent criticism of the activities of the security services.

Privacy campaigners have accused surveillance agency GCHQ of using "unlawful hacking" to spy on citizens.

But Mrs May said this was "nonsense".

The government has defended its use of surveillance powers since documents leaked by former Central Intelligence Agency technical worker Edward Snowden were first published by the Guardian and Washington Post newspapers in 2013.

Campaign group Privacy International has said the documents had detailed the many ways that GCHQ was spying on people, accusing the UK intelligence services of violating the European Convention on Human Rights, which guarantees a right to privacy and to freedom of expression.

'Life and death'

Charles Farr, the director general of the Office for Security and Counter-Terrorism, said last week that GCHQ can legally snoop on British use of Google, Facebook and web-based email without specific warrants because the firms are based abroad.

Unrestrained, unregulated government spying of this kind is the antithesis of the rule of law and government must be held accountable for their actions Eric King, Privacy International

But Mrs May said: "Some people have alleged that GCHQ is exploiting a technical loophole in legislation that allows them to intercept external communications - that is, communications either sent or received outside the UK - at will and without authorisation. This is... nonsense."

She also denied that the security services were able to ask "their counterparts overseas to undertake activity that would be unlawful if they conducted it themselves".

Surveillance powers were only ever used "when they are necessary and proportionate", she said.

But Mrs May warned that it would be "cavalier and reckless" to let the public know details of which terrorist plots had been thwarted by the security services.

"Considerable" threats to UK security were developing with the emergence of militant group Isis, the collapse of Syria, the activities of Boko Haram in Nigeria, and the "expanding scope" of cyber crime, the home secretary said.

She concluded by renewing a call to change the law to hand the security services more powers to scrutinise online communication - a bid that has previously been blocked by the Liberal Democrats.

"We have to make sure that the capabilities can only be used with the right authorisation and with appropriate oversight," she said.

"But this is quite simply a question of life and death, a matter of national security. We must keep on making the case until we get the changes we need."

Image copyright Reuters Image caption Revelations by Edward Snowden show that GCHQ and NSA snooping goes too far, say campaigners

The Investigatory Powers Tribunal, which monitors whether the UK's spying laws are being observed, is currently investigating claims put to it by Privacy International.

The group argues that programmes by GCHQ and its US counterpart NSA, uncovered by Mr Snowden, let the agencies listen in on the public via computer microphones, watch through webcams and scoop up detailed web browsing histories.

Eric King, deputy director of Privacy International, said the surveillance was the modern equivalent of the government entering someone's house and reading their diary, correspondence and journals.

The freedom GCHQ and the NSA had to carry out surveillance was equivalent to "covert, complete, real-time physical and electronic surveillance", he said.

"Arbitrary powers such as these are the purview of dictatorships, not democracies," he said. "Unrestrained, unregulated government spying of this kind is the antithesis of the rule of law and government must be held accountable for their actions."