"Anarchist elements are actively organizing to protest the summit," says the confidential report, released to Southam News under the Access to Information Act.
The hemispheric meeting will bring together the leaders of 34 countries, including Canada, to discuss free trade and investment.
Protesters argue corporate globalization and the unbridled pursuit of profit are seriously eroding the rights of workers, feeding social injustices and damaging the environment.
For almost a year, activists have been preparing for the summit through Internet sites, e-mail, meetings and training camps.
While many demonstrators intend to express opposition peacefully, some are calling for "direct action" to achieve goals.
At previous global summits, the more extreme members of the anti-globalization movement have smashed windows, set fires and trashed stores.
For several months, some activists have urged protesters to shut down the Quebec City meetings.
In an interview, CSIS spokesperson Dan Lambert said "the radical anarchist fringe" has demonstrated a willingness to use violence.
"Given the situation, there is a likelihood of disruptive activity."
The CSIS report stresses most protest groups will "conduct their activities in a peaceful, legitimate manner."
But the possibility of violence is heightened by the efforts of a "loosely organized cluster" of alliances and individuals known as the Black Bloc to establish an anarchist presence in Quebec City and form "affinity groups" of five to 10 individuals.
Pamphlets distributed in Montreal suggested these groups handle various arrangements including medical services, projectile attacks, emergency intervention, human blockade, police surveillance and communication.