Marcel Savard, the force's assistant director, told a commission looking into the student protests of 2012 that police underestimated demonstrators outside a provincial Liberal meeting in May 2012.
He also confirmed fences erected to keep demonstrators away from the gathering in the central Quebec town of Victoriaville were inadequate and there was a breakdown in the communication of intelligence information.
Savard said police have learned valuable lessons from the experience.
He said maintaining the peace is a shared responsibility between police and demonstrators and that such weapons as rubber bullets were not used indiscriminately.
Students protested last year in opposition to tuition fee increases imposed by the provincial government, which argued fees had not been raised in years. Many demonstrations were held in Montreal and some of them turned violent.