"I thought we were all supposed to speak out," he told The Washington Post in Tuesday's editions. "Isn't that what this country is about?"
In a 20-minute address to graduates at the Long Island school on Sunday, the novelist criticized Mr. Bush's tax cuts, anti-terrorism policies and the Patriot Act, but focused mainly on what he called Mr. Bush's "untrue" stories about the war in Iraq.
"One story he told was that the country of Iraq had nuclear and biological and chemical weapons of mass destruction and was intending shortly to use them on us ... but it was not true," Doctorow said.
"Another story was that the Iraqi dictator, Saddam Hussein, was in league with the terrorists of al Qaeda, and that turned out not to be true. But anyway we went off to war on the basis of those stories."
That led to a torrent of boos and catcalls that forced Doctorow to stop talking. Hofstra President Stuart Rabinowitz stepped to the podium and asked the audience to let him finish.
A Hofstra spokeswoman said Monday that e-mail and other reaction to Doctorow's remarks had been "all over the map," but on balance, "more angry comments than positive."
"We feel that first and foremost, the commencement is a celebration of our graduates and their accomplishments," said Melissa Connolly, assistant vice president for university relations. "We regret that the contents of his speech diminished the day for some of our graduates and their families."
Doctorow, 73, a National Book Award and PEN/Faulkner prize winner, received an honorary doctorate in humane letters.
He told the Post that his speech made the point to graduates that "simply because something was said by authority did not mean it shouldn't be questioned. I think it was entirely appropriate."