LONDON : The head of Britain's domestic intelligence agency on Tuesday warned there were thousands of extremists in the country who regard the public as "legitimate targets", and defended the use of snooping technology.

MI5 chief Andrew Parker said the agency was "tackling threats on more fronts than ever before" during a speech at the Royal United Services Institute in London, his first since taking over in April.

The director general said he expected one or two attempts at major acts of terrorism in Britain each year.

He highlighted Al-Qaeda and its affiliates as presenting "the most direct and immediate threats", and said there was "good reason to be concerned about Syria".

But he dismissed claims that electronic eavesdropping agency GCHQ was listening in on everyday communications as "utter nonsense" and defended the use of snooping technology, as revealed in US security documents leaked by former CIA operative Edward Snowden.

"Far from being gratuitous harvesters of private information, in practice we focus our work very carefully and tightly against those who intend harm," he said, adding safeguards were in place to protect citizens.

"In some quarters there seems to be a vague notion that we monitor everyone and all their communications, browsing at will through people's private lives for anything that looks interesting. That is, of course, utter nonsense."

He blasted the leak, saying it caused "enormous damage to make public the reach and limits of GCHQ techniques."

"Such information hands the advantage to the terrorists," he added.

GCHQ, which stands for Government Communications Headquarters, is based in southwest England and monitors communications worldwide for intelligence purposes.

Documents leaked by former US National Security Agency contractor Snowden were reported by the Guardian newspaper in June to show that GCHQ was using data harvested by the secret US PRISM surveillance programme.