"He was a voice of certainty in a world that was growing more and more uncertain, and through it all he never lost his integrity," said Obama.
Former President Clinton also lauded the broadcasting legend at the memorial service at Lincoln Center in New York, as did Cronkite contemporaries such as Tom Brokaw and Bob Scheiffer. Cronkite, who was the anchorman of CBS' nightly newscast from 1962 to 1981, died in July.
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Cronkite set "a standard of honesty, integ and responsibility to which so many of you have committed your careers," Obama told the crowd, which included many journalism luminaries.
The president is expected back at the White House by the afternoon, well in advance of his major speech on health care.
President Clinton, one of the first speakers, noted Obama's appearance.
"I think it says a lot about what Walter Cronkite meant to all of us that one of the most important days in his young presidency, and a very important day for our nation, that President Obama, with his big speech tonight still came to new york to honor Walter Cronkite," said Clinton to rousing applause.
He added, "Since Walter's not doing the news anymore, and he doesn't have to be so objective, I can only say that he and all the rest of us wish you well tonight."
"Margaritaville" singer Jimmy Buffett took the stage next, and noted that Clinton was "a hard act to follow."
Tom Brokaw later paid tribute to Cronkite, saying, to his children, "I want to thank you for sharing him with the rest of the us. In so many ways Walter Cronkite was our dad. He showed us the way to be good journalists, good fathers and good citizens."
Katie Couric, Andy Rooney, Bob Schieffer and Buzz Aldrin also spoke.
"Story telling was Walter's passion, just as flying in space was mine," said Aldrin.
The U.S. Marine Corps band opened the ceremony, and percussionist Mickey Hart performed a tribute. Jazz man Wynton Marsalis was also expected to play.