NEW YORK - National Security Agency leaker Edward Snowden on Wednesday thanked supporters who launched a campaign for his pardon and said for the sake of democracy, future whistleblowers must not be silenced.

Speaking by video link from Moscow, where he has been in exile since 2013, Snowden said that while the Founding Fathers created checks and balances to guard against government abuses, "whistleblowers, acting in the public interest, often at great risk to themselves, are another check on those abuses of power, especially through their collaboration with journalists."

He said whistleblowing "is democracy's safeguard of last resort, the one on which we rely when all other checks and balances have failed and the public has no idea what's going on behind closed doors."

The 33-year-old addressed a New York City news conference where advocates from the American Civil Liberties Union, Human Rights Watch and Amnesty International announced an online petition drive to urge President Barack Obama to pardon Snowden before he leaves office.

The supporters called Snowden a hero for exposing the extent of government surveillance by giving thousands of classified documents to journalists.

"Cases like Edward Snowden's are precisely why the presidential pardon power exists," ACLU Executive Director Anthony Romero said. "There is widespread consensus that Edward Snowden's actions catalyzed an unprecedented debate about the proper limits of government surveillance, and his actions resulted in widespread reforms both in law and in technology that protect Americans and individuals across the globe."

The Obama administration has urged Snowden to return to the U.S. and face trial.

Justice Department spokesman Marc Raimondi said Wednesday: "It is important to remember, Mr. Snowden is not a whistleblower. He is accused of leaking classified information and there is no question his actions have inflicted serious harms on our national security."

Naureen Shah, Amnesty International's director of human rights for the U.S., brandished a photo of Snowden and said, "I think it's no exaggeration to say that this man changed the world."

Because of Snowden, Shah said, human rights defenders "are more empowered than perhaps ever before to challenge surveillance laws and surveillance tools that have as their purpose and their effect the crushing and controlling of dissent all over the world."

The launch of the presidential pardon campaign comes two days before Oliver Stone's biopic "Snowden" opens. Asked whether the film might help the case for his pardon, Snowden said he hopes rather that the movie puts issues of government overreach before a new audience.

"The story of 2013 I think quite centrally and about the film is that from time to time we see that governments begin to redraw the boundaries of our rights behind closed doors." Snowden said.

Public figures supporting the request for a pardon include Wikipedia founder Jimmy Wales, philanthropist George Soros, Pentagon Papers leaker Daniel Ellsberg and writer Joyce Carol Oates.