(CNN) -- Sweeping up Americans' telephone records and monitoring Internet activity from overseas are "modest encroachments on privacy" that can help U.S. intelligence analysts disrupt terror activity, President Barack Obama said Friday.

"Nobody is listening to your telephone calls," he reassured Americans worried about the prospect of government agents listening in on private conversations.

The public commentary on what, until Wednesday, had been top secret programs followed the first public confirmations of the program Thursday by Director of National Security James Clapper, who described the programs as limited, legal and crucial to national security.

Just three months ago, Clapper denied in a congressional hearing that the government was engaged in the mass collection of data on millions of Americans.

In statements issued Thursday, Clapper acknowledged that the United States is collecting domestic telephone records to ferret out terrorist plots.

He also indirectly confirmed the existence of a program using data from some of the world's biggest online services companies -- including Microsoft, Apple, Google, Yahoo and Facebook -- to monitor the online activities of non-U.S. residents overseas.

But he said media reports suggesting widespread surveillance of U.S. citizens and their telephone and online habits have it all wrong, and such reports seriously endanger critical programs to protect Americans from terrorists.

He called disclosure of the Internet monitoring program "reprehensible" and said news of the telephone-monitoring program "threatens potentially long-lasting and irreversible harm to our ability to identify and respond to the many threats facing our nation."

The revelations have reopened impassioned debate over how much intelligence is necessary to fight terrorism and when it amounts to dangerous government overreach.

Telephone monitoring

A British newspaper, the Guardian, published on Wednesday a top secret order from an intelligence court that required Verizon Business Network Services to give telephone records detailing the time, location and telephone numbers involved in domestic calls from April 25 to July 19. The order doesn't allow authorities to listen in on the calls.

Sen. Dianne Feinstein, chairwoman of the Senate Intelligence Committee, later said the order is a standard three-month renewal of a program that has been under way for years. The Electronic Frontier Foundation and other analysts said similar orders undoubtedly are in effect for other companies.

Clapper, who denied during a March 12 congressional hearing that the government was engaged in mass data-collection practices involving Americans, blasted the Guardian report, saying it "omits key information" regarding "safeguards that protect privacy and civil liberties."

He said the program operates within the law, respects Americans' privacy and is crucial to preventing attacks. Most of the records are never looked at, and those that are can be reviewed only with judicial approval, he said.

"The collection is broad in scope because more narrow collection would limit our ability to screen for and identify terrorism-related communications. Acquiring this information allows us to make connections related to terrorist activities over time," he said.

Clapper said both Congress and courts have reviewed and authorized the law.

But several congressional critics have complained that the government has made significant use of broad interpretations of the law made in secret by the Foreign Intelligence Surveillance Court, the judicial body charged with overseeing the government's use of the law.

"These reports are deeply concerning and raise questions about whether our constitutional rights are secure," Patriot Act co-author Rep. James Sensenbrenner wrote in a letter to Attorney General Eric Holder.

Sensenbrenner, a Wisconsin Republican, said he was "extremely disturbed by what appears to be an overbroad interpretation of the act."

Online monitoring

A day after the initial Guardian report on the Verizon court order, the Guardian and the Washington Post published reports alleging the NSA is able to tap into data held by some of the world's biggest online services companies as it hunts for terrorists.

The program, which the newspapers said is called PRISM, reportedly allows NSA analysts access to computers at Microsoft, Google and other companies to extract details of customer activities, including "audio and video chats, photographs, e-mails, documents" and other materials, The Post reported.

The Post reported PRISM, founded in 2007, has become the leading source of raw material for the NSA.

Clapper did not directly confirm the program's existence but acknowledged that the Post and Guardian stories "refer to collection of communications pursuant to Section 702 of the Foreign Intelligence Surveillance Act."

That section of the law authorizes intelligence agencies to collect information on non-U.S. residents as part of efforts to gather foreign intelligence.

He said the program cannot be used to target anyone inside the United States or U.S. citizens anywhere in the world and includes "extensive procedures, specifically approved by the court, to ensure that only non-U.S. persons outside the U.S. are targeted, and that minimize the acquisition, retention and dissemination of incidentally acquired information about U.S. persons."

"Information collected under this program is among the most important and valuable foreign intelligence information we collect and is used to protect our nation from a wide variety of threats," Clapper said Thursday.

"The unauthorized disclosure of information about this important and entirely legal program is reprehensible and risks important protections for the security of Americans," he said.

While the Post initially reported that the companies "knowingly participate" in the program, it later updated the online version of its story to remove that contention.

In its stead, the newspaper described a classified report indicating that NSA analysts were allowed to send "content tasking instructions directly to equipment installed at company-controlled locations."

Microsoft issued a statement Thursday saying it does not participate in any national security data gathering program. Facebook and Google said they do not give government agencies direct access to their servers.

Apple spokesman Steve Dowling said his company has never heard of PRISM.

Verizon declined comment to the media Thursday, but company Vice President Randy Milch, in a note to employees, did say the Guardian's story may spur the company to respond in defiance of a promise of secrecy.

'Beyond Orwellian'

Civil rights groups were appalled.

The American Civil Liberties Union described the programs as "beyond Orwellian."

The Electronic Frontier Foundation, which has been fighting the government for years over surveillance practices, called for a full-scale investigation similar to 1975's Church Committee, which led to an overhaul of intelligence practices following widespread abuses.

"It's time for a full accounting of America's secret spying programs -- and an end to unconstitutional surveillance," the group said on its blog.

Similar efforts have floated to the surface for years.

After the 2001 terrorist attacks, President George W. Bush's administration authorized the NSA to conduct wireless surveillance of international phone calls that included at least one person believed to be an al Qaeda-affiliated terrorist.

The program was later ruled unconstitutional.

Another Bush-era program, the "total information awareness" program, would have combined vast government databases into a data-mining effort to look for suspicious patterns revealing potential criminal or terrorist activity. It was official dropped amid intense outcry from civil libertarians.

Also, in 2003, a former AT&T technician made a splash when he alleged that the telecom was routing all Internet traffic into a special NSA-controlled room in San Francisco.

Last year, Wired magazine wrote about a huge data center being built for the NSA in Utah. Officials have officially labeled the center as part of the nation's effort to fight cyberattacks, but the Wired article, citing former NSA officials and others, said it was part of a huge surveillance program that would take in and analyze untold amounts of data.

Necessary measures

Opinions were also strong on the other side of the debate.

Conservative Sen. Lindsey Graham, a Republican from South Carolina, concurred with Feinstein, a California liberal, that the methods are necessary to prevent terrorism.

"Terrorists will come after us if they can," Feinstein said, "and the only thing that we have to deter this is good intelligence to understand that a plot has been hatched and to get there before they get to us."

Graham said that as a Verizon customer, "it doesn't bother me one bit for the National Security Agency to have my phone number."

When Senate Appropriations Committee Chairwoman Sen. Barbara Mikulski read the news Thursday morning, the Maryland Democrat said, "It was like, 'Oh, God, not one more thing ... where we're trying to protect America and then it looks like we're spying.' "

Obama takes heat

The reports have increased scrutiny of Obama's record on balancing citizens' right to privacy and the government's efforts to combat terrorism.

Days after taking office in 2009, Obama vowed, "Transparency and the rule of law will be the touchstones of this presidency." But on Thursday, the left-leaning Huffington Post on its home page conflated an image of Obama with one of his predecessor, Bush, in a move to criticize the secret surveillance both administrations have been accused of.

A New York Times editorial said "the administration has now lost all credibility" when it comes to overreaching in the name of fighting terrorism.

White House spokesman Josh Earnest stressed the importance of ensuring that "we have the tools we need to confront the threat posed by terrorists (and to) protect the homeland." He called it Obama's top domestic security priority.

"But ... we need to balance that priority with the need to protect civil liberties and constitutional rights of American people," he said.

"And that is the subject of a worthy debate."