19 March 2014
Last updated at 09:53
By Jane Wakefield Technology reporter Whistleblower Edward Snowden has appeared as a surprise guest at the Ted (Technology, Entertainment and Design) conference in Vancouver.
Speaking via a robotic screen from an undisclosed location in Russia, the fugitive promised that there are more revelations to come.
He urged tech firms to protect users by encrypting all web pages.
Web inventor Sir Tim Berners-Lee joined him on stage, describing him as a "hero".
Ex-NSA (National Security Agency) employee Mr Snowden has leaked thousands of secret documents to newspapers, including the Washington Post and the Guardian, over the past year.
They include revelations about how the NSA gathers vast amounts of information, as well as specific details about its operations.
Mr Snowden told the Ted audience: "Some of the most important reporting to be done is yet to come."
He talked about how the agency had overstepped its legal authority.
"The NSA violated their own rules thousands of times in a year. They intercepted all the calls in Washington DC by accident. And it turns out the chairman of the Senate intelligence committee had no idea until the Washington Post contacted her for comment. What does that say about the state of oversight in American intelligence?"
He said that technology firms must do more to ensure the privacy of their users.
"The biggest thing that internet companies can do to protect their users is to put encryption on every page you visit.
"If you buy the book 1984 from Amazon, intelligence agencies from around the world can see that."
'They want me dead' Ted curator Chris Anderson asked him why people should care about their privacy.
"People should be able to pick up the phone or send a text message or buy a book online without wondering how these events will look to intelligence agencies," Mr Snowden replied.
The US government accuses its ex-agent of stealing up to 1.7 million top secret documents. It says that his actions have hugely damaged national security.
The NSA was invited to come to the Ted debate but was unable to provide a representative for "logistical reasons", according to Mr Anderson.
Mr Snowden was asked whether the price he had paid for exposing some of the programmes that the NSA was working on had been worth it.
"It is no mystery that there are governments out there that want to see me dead. But I go to sleep thinking what can I do for the American people. I don't want to harm my government but they cannot ignore due process," he said.
Mr Anderson asked the Ted audience to vote on whether Mr Snowden's actions were heroic or reckless. The vast majority chose the first option.
Magna Carta Sir Tim Berners-Lee, who is also speaking at Ted, joined Mr Snowden on stage, offering him a virtual high-five and described him as a hero.
In response, Mr Snowden gave his stamp of approval to Sir Tim's plans for a Magna Carta for the internet which would enshrine web freedom as a right.
"A Magna Carta is exactly what we need. We need to encode our values in the structure of the internet. By engaging the people who rely on it every day we will get a better internet and build a future better than we can imagine," he said.
"My generation grew up in the internet but I never expected to be defending it in such a practical way or representing it as an avatar."
Without it, he might not have been able to fight his own David v Goliath battle, he said.
"It proves the power of the individual to go head to head with the most powerful intelligence agency in the world and win."
Mr Snowden has spoken via the internet at a variety of events in recent months, most recently at the SXSW festival in Austin, Texas.
When asked what it felt like to be back on North American soil, albeit virtually, he joked: "Canada is warmer than I expected."
