Twelve men and five youths said to have been inspired by al Qaeda were arrested in the operation involving hundreds of officers, authorities said.

The group was "planning to commit a series of terrorist attacks against solely Canadian targets in southern Ontario," Royal Canadian Mounted Police Assistant Commissioner Mike McDonell said at a news conference.

"This group took steps to acquire three tons of ammonium nitrate and other components necessary to create explosive devices," he said. (Watch police chief describe how suspects got bomb materials – 0:36)

"To put this in context, the 1995 bombing of the Murrah Federal Building in Oklahoma City that killed 168 people took one ton of ammonium nitrate."