A spokesman for the organizers said Coulter was advised against appearing after about 2,000 "threatening" students crowded the entrance to Marion Hall, posing a security threat.
"It would be physically dangerous for Ann Coulter to proceed with this event," said conservative political activist Ezra Levant inside the hall. "This is an embarrassing day for the University of Ottawa and their student body . . . who chose to silence her through threats and intimidation."
A protest organizer, international studies student Mike Fancie, said he was pleased they were able to stop Coulter from speaking.
"What Ann Coulter is practicing is not free speech, it's hate speech," he said. "She's targeted the Jews, she's targeted the Muslims, she's targeted Canadians, homosexuals, women, almost everybody you could imagine."
The announcement of the cancellation was greeted with shouts of "Shame" and "We want Ann" from about 100 people inside the hall. Outside protesters mockingly chanted "Goodbye Ann Coulter."
About 10 Ottawa police cars were called to the scene, but there were no incidents.
Coulter expressed her outrage, calling the University of Ottawa a "bush league" institution in an interview for The Washington Times.
"This has never happened before," she told the newspaper. "I go to the best schools, Harvard, the Ivy League and those kids are too intellectually proud" to threaten speakers.
Levant blamed the bedlam on university academic vice-president Francois Houle, who had written Coulter to warn her that Canadian laws make provisions for hate speech.
"Promoting hatred against any identifiable group would not only be considered inappropriate, but could in fact lead to criminal charges," he warned her in the letter, which Coulter quickly leaked to the media.
The university has refused to comment since. Levant said Houle's advice to Coulter had emboldened students to block her appearance.
Coulter, a best-selling author and syndicated columnist, was in the middle of a three-city tour of Canada, which began at the University of Western Ontario in London on Monday, and ends in Calgary on Thursday.
The event in London went without incident, but not without controversy.
When answering questions from students, Coulter told a 17-year-old Muslim student to "take a camel" instead of the flying carpet she has previously suggested Muslims use for transportation. Coulter later told CTV that the "camel" remark was a joke.