Ellsberg, an ex-Marine and military analyst, said he held out hope that exposing alleged lies by the Bush administration could still avert an unjust war. He warned that whistleblowers may face ruin of their careers and marriages and be incarcerated.
"Don't wait until the bombs start falling," Ellsberg said at a Tuesday press conference in Washington. "If you know the public is being lied to and you have documents to prove it, go to Congress and go to the press."
Ellsberg did not leak the Pentagon Papers to the New York Times until 1971, although he says he had information in the mid-1960s that he now wishes he had leaked then.
"Do what I wish I had done before the bombs started falling" in Vietnam, Ellsberg said. "I think there is some chance that the truth could avert war."
The thousands of pages in the Pentagon Papers showed the government's secret decision-making process on Vietnam since the end of World War II. Their publication -- the government sued and lost to prevent it -- is widely credited with helping to turn public opinion against the war in Southeast Asia.
Ellsberg's press conference comes a little more than a week after the London Observer reported on what it said is a top-secret memo showing that the United States planned to spy on U.N. delegates to gain an advantage in the debate over Iraq.
The Observer reported the electronic memo dated Jan. 31, by high-ranking National Security Agency operative Fank Koza, says the agency is "mounting a surge" of intelligence activities mostly focused on U.N. Security Council members for "information that could give U.S. policy-makers an edge in obtaining results favorable to U.S. goals or to head off surprises."
NSA spokesman Patrick Weadon wouldn't comment on the authenticity of the e-mail memorandum. "We have no statement," he said.
U.N. ambassadors have mostly shrugged off the memorandum as reflecting the regular course of business at the United Nations.
Ellsberg said this story on spying at the United Nations is potentially more significant than the Pentagon Papers because it comes before a war has begun and it shows a desperate Bush administration. "This leak is potentially more significant than the release of the Pentagon Papers, since it is extraordinarily timely," Ellsberg said.
This past Sunday, the Observer reported that an employee at the top-secret British Government Communications Headquarters had been arrested following publication of the story. Ellsberg said reporters at the Observer told him the 28-year old woman arrested was not the source of the leak.
A second U.S. diplomat resigned yesterday in protest against the Bush administration's war stance. John H. Brown, who served in the diplomatic corps since 1981, said Bush's disregard for the views of other nations was giving birth to "an anti-American century." Last month, a senior U.S. diplomat based in Athens, political counselor John Brady Kiesling, resigned with similar complaints.
Last week, Mohamed ElBaradei, director general of the International Atomic Energy Agency, rejected a Bush administration claim that Iraq had tried to purchase high-strength aluminum tubes to use in centrifuges for uranium enrichment.