Extensive US surveillance efforts had helped stop "dozens" of possible attacks, according to the head of the US' National Security Agency, who warned that making details of the top-secret programs public had compromised national security.
In his first appearance before Congress since an NSA contractor lifted the veil on the agency's broad monitoring of phone and internet data, General Keith Alexander defended the program as an essential tool in the fight against terrorism.
"It's dozens of terrorist events that these have helped prevent," the NSA director told a US Senate committee. "Both here and abroad, in disrupting or contributing to the disruption of terrorist attacks."
Relying on documents from NSA contractor Edward Snowden, Britain's Guardian newspaper and the Washington Post revealed last week the vast US government effort to monitor phone and internet data at nine technology companies including Google and Facebook.
Alexander said the disclosures, which have sparked a criminal investigation and an internal Obama administration review of the potential national security damage, had jeopardised safety in the United States and elsewhere.
"Great harm has already been done by opening this up," Alexander said. "There is no doubt in my mind that we will lose capabilities as a result of this and that not only the United States but those allies that we have helped will no longer be as safe as they were two weeks ago."
Snowden, who traveled to China-ruled Hong Kong before the program was made public, said he planned to stay in the former British colony and fight any effort to bring him back to the United States for criminal proceedings.
"I am not here to hide from justice. I am here to reveal criminality," Snowden told the South China Morning Post.
"My intention is to ask the courts and people of Hong Kong to decide my fate," Snowden said. "I have had many opportunities to flee Hong Kong, but I would rather stay and fight the United States government in the courts, because I have faith in Hong Kong's rule of law."
Hong Kong, which has a degree of autonomy from Beijing, has an extradition agreement with the United States that has been exercised on numerous occasions. But Snowden has not been publicly charged so far and the United States has not filed for his extradition.
Snowden, who had been working at an NSA facility as an employee of Booz Allen Hamilton, has drawn a mix of condemnation and praise for the revelations. He told the Hong Kong newspaper: "I'm neither traitor nor hero. I'm an American."
The controversy over the program has renewed the debate about the balance between privacy rights and security concerns in the United States in the aftermath of the September 11, 2001, attacks. Alexander said the NSA operated with that balance in mind.
"I want the American people to know that we're trying to be transparent here and protect civil liberties and privacy but also the security of this country," he said.
But he added, "I would rather take a public beating and people think I'm hiding something, than jeopardise the security of this country."
He denied the agency was doing anything wrong or harming civil liberties, but said it was struggling to offer details without revealing classified information.
"This is not us doing something under the covers," Alexander said. "We want to tell you what we're doing, and tell you that it's right, and let the American people see this."
Alexander promised to make details of the thwarted attacks available to the public within the next week. Officials said last week the email surveillance program played a role in foiling a 2009 Islamist militant plot to bomb the New York City subway system.