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

But civil liberty groups immediately criticized the collection of information.

“The stories published over the last two days make clear that the NSA – part of the military – now has direct access to every corner of Americans’ digital lives,” ACLU Deputy Legal Director Jameel Jaffer said. “Unchecked government surveillance presents a grave threat to democratic freedoms. These revelations are a reminder that Congress has given the executive branch far too much power to invade individual privacy, that existing civil liberties safeguards are grossly inadequate, and that powers exercised entirely in secret, without public accountability of any kind, will certainly be abused.”

The government began collecting telephone records after the Sept. 11, 2001, terrorist attacks. Though civil rights groups had long suspected the practice continued, there had been no evidence.

The order for the phone records falls under Section 215 of the Patriot Act, which authorizes the government to make broad demands on telephone providers for information about calls.

As a senator, Obama had supported changes to the Patriot Act that would have required the government to convince a judge that the records they are seeking have some connection to a suspected terrorist or spy. But as president, he has signed its re-authorization.

“In his first inaugural address, President Obama stated that, ‘as for our common defense, we reject as false the choice between our safety and our ideals,’” said Stephen B. Wicker, a Cornell University professor and expert on digital security. “He has apparently changed his mind.”

Holder, previously scheduled to testify on Capitol Hill, said that the Obama administration has kept Congress fully informed about the program and that he would be willing to provide more details in a closed hearing.

In a show of bipartisan unity, Sens. Dianne Feinstein, D-Calif., chairwoman of the Senate Intelligence Committee, and Saxby Chambliss of Georgia, the committee’s top Republican, told reporters that Congress was aware of the surveillance program.

“As far as I know, this is the exact three-month renewal of what has been the case for the past seven years,” Feinstein said. “Therefore, it is lawful. It has been briefed to Congress.”

Their committee sent letters to senators about the program in 2010 and 2011, informing them that it was on the verge of renewal and that they could review information about it on a classified basis.

“Every member of the United States Senate has been advised of this and to my knowledge we’ve not had any citizen who has registered a complaint relative to the gathering of this information,” Chambliss said.

Still, the program has its share of critics on Capitol Hill.

“The United States should not be accumulating phone records on tens of millions of innocent Americans,” said Sen. Bernie Sanders, a Vermont independent who voted against the Patriot Act when it was first enacted in 2001 and when it was reauthorized in 2006 and 2011.

Sen. Rand Paul, R-Ky., said that “after the revelations that the Internal Revenue Service targeted political dissidents and the Justice Department seized reporters’ phone records, it would appear that this administration has now sunk to a new low.”

The order was signed by a judge from the secret court that oversees domestic surveillance, the Foreign Intelligence Surveillance Court, for a three-month period ending on July 19. Congress established the court, comprised of 11 federal judges, in 1978.

The court appears to be a rubberstamp for the administration, according to the court’s own records. It received 1,789 applications to conduct electronic surveillance during 2012, according to the court’s most recent public report. It did not deny any applications.

The order is one of the most highly classified U.S. government documents ever to become public without authorization, making a Justice Department leak investigation certain.

Michael Doyle and Jonathan S. Landay of the Washington Bureau contributed to this report.