The Royal Canadian Mounted Police, who conducted the sweep alongside domestic anti-terrorism units, did not reveal the nature of the charges, although some local reports said they related to an explosives plot.
The individuals are being held at a heavily guarded police station in Pickering, a suburb east of the city.
The Toronto Star newspaper reported that the Canadian Security Intelligence Service (CSIS) had been monitoring the group since 2004 and launched an investigation last year.
More than 400 officers are involved in the investigation and more arrests would take place, said Corporal Michele Paradis.
"The investigation is ongoing. Most of the police services throughout the greater Toronto area have been involved," she said.
Canadian intelligence services warned last week that the country faced an increasing threat from "homegrown terrorists".
Jack Hooper, deputy director of operations at CSIS, said the service was trying to keep track of 350 "high-level" targets as well as 50 to 60 organisations thought to be linked to groups such as al-Qaeda.