WASHINGTON — President Obama on Friday sought to take control of the roiling debate over the National Security Agency’s surveillance practices, releasing a more detailed legal justification for domestic spying and calling for more openness and scrutiny of the N.S.A.’s programs to reassure a skeptical public that its privacy is not being violated. 

“It’s right to ask questions about surveillance, particularly as technology is reshaping every aspect of our lives,” Mr. Obama said, adding: “It’s not enough for me, as president, to have confidence in these programs. The American people need to have confidence in them as well.” 

But at a time when leaks by the former N.S.A. contractor Edward J. Snowden have exposed the agency’s expansive spying both inside the United States and abroad to an unprecedented degree of scrutiny, Mr. Obama showed no inclination to curtail secret surveillance efforts. Rather, he conceded only a need for greater openness and safeguards to make the public “comfortable” with them. 

In meeting threats to the country, Mr. Obama said, “we have to strike the right balance between protecting our security and preserving our freedoms.” And while he said that the programs were valuable and that he was confident they had not been abused, he acknowledged that people “may want to jigger slightly” that balance. 

Mr. Obama made his remarks at a wide-ranging news conference on the eve of his departure for a week’s vacation. He responded to questions on issues like the coming appointment of a new Federal Reserve chairman, the carrying out of his health care law, his relationship with President Vladimir V. Putin of Russia, and the current status of Al Qaeda. But he began with a lengthy statement about surveillance, and that was the focus of the nearly hourlong news conference. 

Critics of the electronic spying brought to light by Mr. Snowden’s leaks said the president’s approach was insufficient. Anthony D. Romero, the executive director of the American Civil Liberties Union, said that a program that collects records of every domestic phone call — which Mr. Obama made clear he intends to keep — must be shut down. 

“What’s clear is that these surveillance programs have gone much further than the president or Congress have ever admitted,” Mr. Romero said. “These initial recommendations from Obama today, albeit welcome, are too little too late. They are not sufficient to address serious concerns about possible violations of the law and about dragnet surveillance.” 

A spokesman for Speaker John A. Boehner, Republican of Ohio, urged Mr. Obama not to let such criticism undermine the N.S.A.’s fundamental capabilities. 

“Transparency is important, but we expect the White House to insist that no reform will compromise the operational integrity of the program,” said the spokesman, Brendan Buck. “That must be the president’s red line, and he must enforce it. Our priority should continue to be saving American lives, not saving face.” 

A clear theme of Mr. Obama’s remarks was that he believed that the public’s understanding of the surveillance programs had been distorted. He portrayed some of Mr. Snowden’s leaks as having been reported in “the most sensationalized manner possible” and parceled out to “maximize attention” in “dribs and in drabs, sometimes coming out sideways.” The result has been misimpressions not merely among the American public, he said, but around the world — a reference to the widespread international criticism of the United States over reports of its surveillance policies. 

“If you are the ordinary person and you start seeing a bunch of headlines saying ‘U.S. Big Brother looking down on you, collecting telephone records, etc.,’ well, understandably people would be concerned,” he said, while also addressing some of his reassurances to those abroad. 

“To others around the world, I want to make clear once again that America is not interested in spying on ordinary people,” he said. “Our intelligence is focused above all on finding the information that’s necessary to protect our people and, in many cases, protect our allies. It’s true we have significant capabilities. What’s also true is we show a restraint that many governments around the world don’t even think to do.” 

In an effort to rebuild public trust, Mr. Obama said he wanted to work with Congress to modify the phone log program, but in what he said would be an “appropriate” way. He listed as examples of those steps establishing more oversight and auditing how the database is used. 

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