Ayers, 64, arrived on a flight to Toronto Sunday night to give a series of lectures and conferences at the University of Toronto's Ontario Institute for Studies in Education on Monday, the Toronto Star reported Tuesday.
Ayers teaches at the University of Illinois-Chicago and has been a supporter of Barack Obama's political career dating back to 1995. However, he was also a founder of the Weather Underground movement, which claimed responsibility for a number of bombings in Washington, including the U.S. Capitol, and New York in the early 1970s.
In a telephone interview with the Star, Ayers said he didn't understand why he was turned back.
"To me it seems quite bureaucratic and not at all interesting," he said. "If it were me I would have let me in. I couldn't possibly be a threat to Canada."
A spokeswoman for the Canadian Border Services Agency would not explain the decision to the Chicago Tribune, but said people can be refused entry for reasons ranging from health and financial issues to perceived security threats.
Jordana Brewster on Paul Walker: 'He was an enormous presence in my life'
Gal Gadot cast as Wonder Woman for 'Batman vs. Superman'