Washington - Mounting revelations about the extent of NSA surveillance have alarmed technology leaders in recent days, driving a renewed push for significant legislative action from an industry that long tried to stay above the fray in Washington.

After months of merely calling for the government to be more transparent about its surveillance requests, tech leaders have begun demanding substantive new restraints on how the National Security Agency collects and uses the vast quantities of information it scoops up around the globe, much of it from the data streams of U.S. companies.

The pivot marks an aggressive new posture for an industry that often has trod carefully in Washington - devoting more attention to blunting potentially damaging actions than to pushing initiatives that might prove controversial and alienate users from its lucrative services.

Six leading technology companies - Facebook, Google, Apple, Yahoo, Microsoft and AOL - sent a letter to Senate leaders Thursday reflecting the sharpening industry strategy, praising the sponsors of a bill that would end the bulk collection of phone records of millions of Americans and create a privacy advocate to represent civil liberties interests within the secretive court that oversees the NSA.

"Transparency is a critical first step to an informed public debate, but it is clear that more needs to be done," said the letter, which was sent to Sen. Patrick Leahy, D-Vt., chairman of the Judiciary Committee and one of the bill's sponsors, as well as three other senators. "Our companies believe that government surveillance practices should also be reformed to include substantial enhancements to privacy protections and appropriate oversight and accountability mechanisms for those programs."

Although historically wary of Washington, the technology industry has been bulking up its political operations in the nation's capital for several years. It took a public stand against the Stop Online Piracy Act, commonly known as SOPA, with a massive Internet protest last year. More recently, tech leaders made a high-profile push in the immigration debate, calling for more visas for foreign-born workers.

The tone of industry reaction to the NSA revelations has grown more aggressive since the first stories appeared in The Washington Post and Britain's Guardian newspaper in June. Companies that initially were focused on defending their reputations gradually began criticizing the government and challenging it in court. Some companies also have worked to harden their networks against infiltration.

A turning point came with Thursday's Washington Post revealing an NSA program that collects massive amounts of user information from Google and Yahoo as it moved among data centers overseas. To some, this amounted to a degree of intrusiveness that, though speculated about by privacy activists, was beyond what many in the industry thought possible.

"Clearly, this is something new and different," said Joseph Lorenzo Hall, the chief technologist at the Center for Democracy and Technology, a Washington-based think tank that receives substantial industry support.

Hall said technology leaders are weary of the revelations. "Right now, it's like, 'Please make it stop!' "

The backlash from the technology industry is particularly striking in light of its once-cozy relationship with President Barack Obama, who got money and votes out of Silicon Valley at historic rates last year.

Although Google's general counsel, David Drummond, issued a statement Thursday expressing "outrage" and "the need for urgent reform," a longtime security engineer for the company better captured the industry's mood in a post on Google Plus, a social networking service.

"Even though we suspected this was happening, it still makes me terribly sad. It makes me sad because I believe in America," wrote engineer Brandon Downey, after cautioning that he was speaking personally and not for Google.

National security officials have rejected criticism of NSA's collection of communications, particularly any suggestion that the agency had scooped up data under presidential authorities to avoid the greater oversight required by under the Foreign Intelligence Surveillance Act.

"NSA conducts all of its activities in accordance with applicable laws, regulations, and policies - and assertions to the contrary do a grave disservice to the nation, its allies and partners, and the men and women who make up the National Security Agency," said a statement issued late Thursday by the agency.

Speaking at an American Bar Association conference in Washington, NSA General Counsel Rajesh De defended the agency's practices.

"The implication, the insinuation, suggestion or the outright statement that an agency like NSA would use authority under Executive Order 12333 to evade, skirt or go around FISA is simply inaccurate," De said. "There is no scandal about the lawfulness of NSA's activities under current law."

For all the mounting frustration within the tech industry, the path ahead is murky. Most of the surveillance bills getting wide circulation on Capitol Hill would not address NSA collection operations in other countries.

"To reform this is going to require passing a law that regulates NSA's operations overseas, and none of the bills do that now," said Jennifer Granick, director of civil liberties at Stanford Law School's Center for Internet and Society.

There also are unanswered legal questions. Some scholars say that the NSA's collection of data from Google, Yahoo and their users might violate the Fourth Amendment's prohibition on illegal search and seizure, even if it happens in foreign countries.

Some privacy activists said technology companies share at least some of the blame for the extent of the government surveillance program. They collect the detailed user data - much of it used to target the advertising that generates company profits - that NSA covets. The companies also have lobbied against laws that would limit data collection in Europe and elsewhere.

"Their business model ultimately makes state surveillance not only possible but more far-reaching," said Jeffrey Chester, executive director of the Center for Digital Democracy, a Washington-based consumer rights group. "It's not just the NSA and the national security apparatus that's responsible for this."