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President Obama sought to reassure Americans that the government is not spying on them or unduly violating their civil liberties, after a spate of press leaks that exposed top-secret federal data-mining programs.

But in remarks to reporters Friday, the president also defended such programs as necessary for national security 鈥� and said he welcomed a public debate over the tradeoffs involved in keeping the public safe while also protecting privacy. 聽

鈥淚 think it's important to recognize that you can't have a hundred percent security and also then have a hundred percent privacy and zero inconvenience,鈥� Mr. Obama said in San Jose, Calif., before heading to southern California to meet with China's president. 鈥淵ou know, we're going to have to make some choices as a society.鈥�

His comments followed the second major leak this week about a classified government program aimed at thwarting terrorism. On Friday, the Washington Post published a story describing how the National Security Agency (NSA) and the FBI are tapping directly into the central servers of nine leading US Internet companies as they track foreign targets, according to a top-secret document obtained by the newspaper.聽A separate article in Friday鈥檚 Wall Street Journal discussed NSA data-mining of purchase information from credit-card companies.

On Wednesday, the British newspaper the Guardian exposed a top-secret court order that requires Verizon to provide the NSA with records on phone calls on an 鈥渙ngoing, daily basis.鈥� It has since come out that other telecommunications providers are subject to the same requirements, and that the 90-day orders are regularly renewed by a special court.

The exposure of key elements of the federal government鈥檚 antiterrorism apparatus put the White House on the defensive, as civil libertarians expressed outrage over what they see as an overzealous interpretation of the Patriot Act. As a senator and presidential candidate with a background in constitutional law, Obama had railed against the aggressive antiterror tactics of President George W. Bush. In a 2005 floor speech, then-Senator Obama decried Americans鈥� lack of legal recourse in court against a government 鈥渇ishing expedition鈥� into private records.

But as president, Obama has continued his predecessor鈥檚 antiterrorism programs, and in some cases 鈥� such as in the use of unmanned aircraft to go after suspected terrorists 鈥� he has expanded them.

In his remarks Friday, Obama acknowledged his evolving view, and said he had strengthened the protections to average citizens.

鈥淵ou know, I came in with a healthy skepticism about these programs,鈥� Obama said. 鈥淢y team evaluated them. We scrubbed them thoroughly. We actually expanded some of the oversight, increased some of the safeguards. But my assessment and my team's assessment was that they help us prevent terrorist attacks.鈥�

On Thursday, Rep. Mike Rogers (R) of Michigan, chairman of the House Intelligence Committee, said the telephone metadata program had thwarted a terrorist attack within the past few years.

In response to the Guardian leak, the administration declassified aspects of the phone-records program, including the fact that it does not allow the government to eavesdrop on phone calls.

鈥淣obody is listening to your telephone calls,鈥� Obama said. 鈥淭hat's not what this program's about.鈥�

The only type of information acquired under the court order is 鈥渢elephony metadata,鈥� including numbers dialed and length of calls, according to a press release from the Office of the Director of National Intelligence. Only specially cleared counterterrorism personnel trained in court-approved procedures may access the records.

鈥淚f the intelligence community then actually wants to listen to a phone call, they've got to go back to a federal judge, just like they would in a criminal investigation,鈥� Obama said in California.

The president said he welcomes the debate over how to balance security and civil liberties, calling it 鈥渉ealthy鈥� for democracy.

鈥淚 think it's a sign of maturity, because probably five years ago, six years ago, we might not have been having this debate,鈥� he said. 鈥淎nd I think it's interesting that there are some folks on the left, but also some folks on the right who are now worried about it who weren't very worried about it when it was a Republican president.鈥�

But Obama also made clear he did not welcome the leaks that led to the exposure of the programs. 聽

鈥淭here鈥檚 a reason why these programs are classified,鈥� he said. 鈥淚f every step that we're taking to try to prevent a terrorist act is on the front page of the newspapers or on television, then presumably the people who are trying to do us harm are going to be able to get around our preventive measures.鈥�

But regarding the two programs exposed this week, that horse has left the barn. Obama issued repeated assurances that the public is protected from any overzealous use of those programs, including the oversight of congressional committees and of federal judges with lifetime appointments who are insulated from political pressure.

Obama spoke less about the Internet data-mining program discussed in Friday鈥檚 Washington Post known as PRISM, which involves such service providers as Google, Yahoo, Facebook, Microsoft, and Skype.

The court-approved program is focused on foreign communications traffic, which often flows through US servers even when sent from one overseas location to another, the Post article says.

But despite government reassurances that Americans are not the target, someone the Post identified as a 鈥渃areer intelligence officer鈥� felt strongly enough about the program to leak its existence.

鈥淔irsthand experience with these systems, and horror at their capabilities,鈥� the article concluded, 鈥渋s what drove a career intelligence officer to provide PowerPoint slides about PRISM and supporting materials to The Washington Post in order to expose what he believes to be a gross intrusion on privacy.鈥�