Daniel Ellsberg called Snowden's leak the most important leak in American history Guardian US

Washington was struggling to contain one of the most explosive national security leaks in US history on Monday, as public criticism grew of the sweeping surveillance state revealed by whistleblower Edward Snowden.

Political opinion was split, with some members of Congress calling for the immediate extradition of a man they consider a "defector" but other senior politicians from both parties questioning whether US surveillance practices had gone too far.

Daniel Ellsberg, the former military analyst who revealed secrets of the Vietnam war through the so-called Pentagon Papers in 1971, described Snowden's leak as even more important and perhaps the most significant leak in American history.

In London, the British foreign secretary, William Hague, was forced to defend the UK's use of intelligence gathered by the US. Other European leaders also voiced concern.

The German chancellor, Angela Merkel, is expected to grill Obama next week, during a much-awaited summit in Berlin. Peter Schaar, Germany's federal data protection commissioner, told the Guardian it was unacceptable for the US authorities to have access to EU citizens' data, and that the level of protection is lower than that guaranteed to US citizens.

In Washington, the Obama administration offered no indication on Monday about what it intended to do about Snowden, who was praised by privacy campaigners but condemned by some US politicians keen for him to be extradited from Hong Kong and put on trial.

The White House made no comment beyond a short statement released by a spokesman for the US director of national intelligence on Sunday. Shawn Turner said Snowden's case had been referred to the Justice Department, and that US intelligence was assessing the damage caused by the disclosures.

"Any person who has a security clearance knows that he or she has an obligation to protect classified information and abide by the law," Turner said.

Snowden disclosed his identity in an explosive interview with the Guardian, published on Sunday. He revealed he was a 29-year-old former technical assistant for the CIA and current employee of the defence contractor Booz Allen Hamilton. Snowden worked at the National Security Agency for the past four years as an employee of various outside contractors, including Booz Allen and Dell.

He left for Hong Kong on 20 May. He chose Hong Kong because "they have a spirited commitment to free speech and the right of political dissent".

In his interview, Snowden revealed himself as the source for a series of articles in the Guardian last week, which included disclosures of a wide-ranging secret court order that demanded Verizon pass to the NSA the details of phone calls related to millions of customers, and a huge NSA intelligence system called Prism, which collects data on intelligence targets from the systems of some of the biggest tech companies.

Snowden said he had become disillusioned with the overarching nature of government surveillance in the US. "The government has granted itself power it is not entitled to. There is no public oversight. The result is people like myself have the latitude to go further than they are allowed to," he said.

"My sole motive is to inform the public as to that which is done in their name and that which is done against them."

Snowden drew support from civil liberty activists and organisations. Ellsberg wrote for the Guardian: "In my estimation, there has not been in American history a more important leak than Edward Snowden's release of NSA material – and that definitely includes the Pentagon Papers 40 years ago".

Thomas Drake, a former NSA executive who famously leaked information about what he considered a wasteful data-mining program at the agency, said of Snowden: "He's extraordinarily brave and courageous."

The Electronic Frontier Foundation, an internet rights group, called for a "new Church committee" to investigate potential government infringements on privacy and to write new rules protecting the public. In the wake of the Watergate affair in the mid-1970s, a Senate investigation led by Idaho senator Frank Church uncovered decades of serious abuse by the US government of its eavesdropping powers. The committee report led to the passage of the Foreign Intelligence Surveillance Act and set up the Fisa courts that today secretly approve surveillance requests.

Both Snowden and the Obama administration appeared to be considering their options on Monday. Hong Kong is unlikely to offer Snowden a permanent refuge, but Snowden could buy time by filing an asylum request, thanks to a landmark legal ruling that has thrown the system into disarray.

For years, Hong Kong has relied on the United Nations refugee agency to handle the bulk of claims. But in March its court of final appeal ruled that the government must independently screen cases. No system for processing the claims is yet in place.

China-watchers also wonder if Beijing would wish to become publicly involved in such a high-profile case – particularly given China's doctrine of non-interference in other countries' domestic affairs, and that it comes days after a meeting between presidents Xi Jinping and Barack Obama, as the countries seek to improve bilateral relations.

In New York, Mayor Michael Bloomberg cancelled at very short notice a planned photo opportunity with the Hong Kong chief executive, Leung Chun-ying. "It would have been a circus, so we decided to catch up with him another time," a mayoral spokesman told the Guardian.

Shares in Snowden's employer, Booz Allen, fell on Monday by 61¢, or 3.4%, in midday trading, a slight recovery from a 5% drop earlier in the session.

In a statement on Sunday, the company said it has employed Snowden for less than three months on a team in Hawaii. It added that it is working with clients and authorities to investigate the leaks. "News reports that this individual has claimed to have leaked classified information are shocking, and if accurate, this action represents a grave violation of the code of conduct and core values of our firm," the statement said.

Booz Allen Hamilton is a consultant to government and corporate clients. About 23% of its revenue, or $1.3bn, came from US intelligence agencies last year. The company has said in SEC filings that security breaches could materially hurt results.

Additional reporting by Matt Williams and Tom McCarthy in New York, and the Associated Press