Toronto ? Canadian authorities thwarted what they believed to be a major terrorist threat on home soil with the arrests of 17 people “inspired by al-Qaida” who had stockpiled three times the amount of explosive used in the 1995 Oklahoma City bombing, the country’s national police announced Saturday.

The Royal Canadian Mounted Police said the suspects – 12 men and five juveniles – were all Canadian residents, mostly from the Toronto area, and were rounded up in raids carried out between Friday afternoon and early Saturday morning.

The arrests, officials said, were the culmination of the largest counterterrorism operation in Canada since the passage of the country’s Anti-Terrorism Act shortly after the 9-11 attacks in the United States.

Authorities declined to identify the targets of the alleged plot, saying only that they were in southern Ontario, where Toronto, Canada’s largest city, and Ottawa, its capital, are located. They did, however, deny a report that Toronto’s mass transit system had been targeted.

Authorities bore down on the suspects after the group successfully secured three tons of ammonium nitrate, a fertilizer that can be used to make explosives, said Mike McDonell, assistant commissioner of the mounted police.

A terror suspect is transferred to a prisoner transportation vehicle Saturday in Pickering, Ontario. Canadian police foiled a homegrown terrorist attack involving three times the amount of explosives used in the Oklahoma City bombing, officials said Saturday.

“It was their intent to use it for a terrorist attack,” McDonell said at a news conference in Toronto. “To put it in context, the 1995 bombing of the Murrah Federal Building in Oklahoma City that killed 168 people was completed with only one ton of ammonium nitrate.”

McDonell said the group “posed a real and serious threat. It had the capacity and intent to carry out these attacks.”

Authorities would not say what triggered their probe, which called on more than 400 officers and investigators from several law enforcement agencies and the intelligence community, or how long they had been watching the suspects. They said they could not disclose too many details because the probe was ongoing.

But they did say the suspects, the oldest of whom is 43 and most of whom are young men, trained together. They come “from a variety of backgrounds,” officials said, and represent all walks of life. Some are students, some work and some are unemployed, they said.

Luc Portelance, assistant director of operations for the Canadian Security Intelligence Service – the country’s spy agency – said the suspects “appear to have become adherents of a violent ideology inspired by al-Qaida,” though investigators had not found a link to the network led by Osama bin Laden.

Officials said the arrests proved that the threat of terrorism was real in Canada – and that efforts to foil it were working.

“These individuals were allegedly intent on committing acts of terrorism against their own country and their own people,” recently elected Canadian Prime Minister Stephen Harper said. “As we have said on many occasions, Canada is not immune to the threat of terrorism. … Today, Canada’s security and intelligence measures worked.”