Top U.K. spies say Snowden leaks did serious damage (CBS News) LONDON -- There was a first in the British Parliament Thursday: their top spies who are rarely seen came to a public hearing. They included the head of the MI5 -- their version of the FBI -- and MI6 -- their CIA. They stepped out of the shadows to sound a warning. Not that long ago, the public wouldn't even have known the names of the people who head Britain's foreign, domestic and electronic eavesdropping services. But at an unprecedented hearing, they said the release of thousands of security documents by former NSA contractor Edward Snowden had done serious damage. In an unprecedented hearing before British Parliament, intelligence chiefs said terror groups changed communications tactics after NSA leaks. "Our adversaries are rubbing their hands with glee," said John Sawers, the head of MI6. Iain Lobban, who runs the GCHQ, Britain's version of the NSA, said: "The cumulative effect of the media coverage -- global media coverage -- will be to make the job that we have far, far harder for years to come." Complete Coverage: NSA surveillance exposed The intelligence chiefs say that since London's bus and subway terror attacks in 2005, their eavesdropping has foiled 34 other plots. But, they say, terror groups are now changing how they communicate over the Internet. On a site CBS News found, a jihadi group diagrammed just how the NSA had been listening in to its chatter. Warn others, it says, spread the word. British security chiefs are now facing the same sort of allegations as their counterparts in the U.S.: that they've cast their security net too wide, that they're out of control and that the legal and political framework under which they're working simply hasn't kept pace with the technology that allows them to do what they now do.