Officials from Clapper's office, the Justice Department, NSA and FBI briefed 27 senators for some two hours late Thursday at a hurriedly convened session prompted by severe criticism and uncertainty about the program.

"The National Security Agency's seizure and surveillance of virtually all of Verizon's phone customers is an astounding assault on the Constitution," said Sen. Rand Paul, R-Ky. "After revelations that the Internal Revenue Service targeted political dissidents and the Department of Justice seized reporters' phone records, it would appear that this administration has now sunk to a new low."

Paul said he will introduce legislation ensuring that the Fourth Amendment rights against unreasonable searches and seizures apply to government search of phone records.

The surveillance powers are granted under the post-9/11 Patriot Act, which was renewed in 2006 and again in 2011. Republicans who usually don't miss a chance to criticize the administration offered full support.

"I'm a Verizon customer. I could care less if they're looking at my phone records. ... If you're not getting a call from a terrorist organization, you got nothing to worry about," said Sen. Lindsey Graham, R-S.C.

The disclosures come at a particularly inopportune time for Obama. His administration already faces questions over the Internal Revenue Service's improper targeting of conservative groups, the seizure of journalists' phone records in an investigation into who leaked information to the media, and the handling of the terrorist attack in Libya that left four Americans dead.

At a minimum, it's all a distraction as the president tries to tackle big issues like immigration reform and taxes. And it could serve to erode trust in Obama as he tries to advance his second-term agenda and cement his presidential legacy.

The Verizon order, granted by the secret FISA court on April 25 and good until July 19, requires information on the phone numbers of both parties on a call, as well as call time and duration, and unique identifiers, The Guardian reported.

It does not authorize snooping into the content of phone calls. But with millions of phone records in hand, the NSA's computers can analyze them for patterns, spot unusual behavior and identify "communities of interest" — networks of people in contact with targets or suspicious phone numbers overseas.

Once the government has zeroed in on numbers that it believes are tied to terrorism or foreign governments, it can go back to the court with a wiretap request. That allows the government to monitor the calls in real time, record them and store them indefinitely.

House Intelligence Chairman Mike Rogers, R-Mich., said that once the data has been collected, officials still must follow "a court-approved method and a series of checks and balances to even make the query on a particular number."

The steps are shrouded in government secrecy, which some lawmakers say should change.

"The American public can't be kept in the dark about the basic architecture of the programs designed to protect them," said Sen. Al Franken, D-Minn.

Verizon Executive Vice President and General Counsel Randy Milch, in a blog post, said the company can't comment on any such court order. He said Verizon take steps to protect customers' privacy, but must comply with court orders. Verizon listed 121 million customers in its first-quarter earnings report this April.

The NSA is sensitive to perceptions that it might be spying on Americans. It distributes a brochure that pledges the agency "is unwavering in its respect for U.S. laws and Americans' civil liberties — and its commitment to accountability."

Emerging from the briefing, Sen. Dianne Feinstein, D-Calif., chairwoman of the Intelligence committee, said the government must gather intelligence to prevent plots and keep Americans alive. "That's the goal. If we can do it another way, we're looking to do it another way. We'd like to."

She said Congress is always open to changes, "but that doesn't mean there will be any."