National Security Agency leaker Edward Snowden says it would "chill speech" and "erode the quality of our democracy" if he serves a long prison term in the United States.
Mr Snowden spoke via a videolink at a New York news conference on Wednesday. Advocates, including Human Rights Watch and Amnesty International, are launching a public campaign to persuade President Barack Obama to pardon him.
Speaking from Moscow, where he is in exile, Mr Snowden said he performed a public service by giving thousands of classified documents to journalists in 2013.
He said whistleblowers are "democracy's safeguard of last resort".
White House spokesman Josh Earnest said on Monday that the president believes Mr Snowden should return to the US to face charges.
He said President Obama's position is that Snowden's leaks harmed national security and put Americans at risk.