TORONTO - A group of Canadian residents arrested in coordinated raids across the Toronto area for “terrorism-related offenses” had planned to blow up targets around southern Ontario, Canadian police said on Saturday.

Mike McDonnell, assistant commissioner of the Royal Canadian Mounted Police, said the group had acquired three metric tons of ammonium nitrate — or three times the amount used in the 1995 bombing in Oklahoma City — as they sought to “create explosive devices.” Police said they had arrested 12 adults and five young people.

“This group posed a real and serious threat,” McDonnell said. “It had the capacity and intent to carry out attacks. Our investigation and arrests prevented the assembly of any bombs and the attacks being carried out.”

Officials showed evidence of bomb making materials, a computer hard drive, camouflage uniforms and what appears to be a door with bullet holes in it at a news conference Saturday morning.

“This group took steps to acquire three tons of ammonium nitrate and other components necessary to create explosive devices,” McDonnell said.

The arrests were made Friday, with some 400 officers involved.

McDonnell said the suspects were either citizens or residents of Canada and had trained together.

“The men arrested yesterday are Canadian residents from a variety of backgrounds. For various reasons they appeared to have become adherents of a violent ideology inspired by al-Qaida,” said Luc Portelance, the assistant director of operations with CSIS — Canada’s spy agency.

Heavily armed police officers ringed the Durham Regional Police Station in the city of Pickering, just east of Toronto, as the suspects were brought in late Friday night in unmarked cars which were drove into an underground garage.

The Toronto Star reported Saturday that Canadian youths in their teens and 20s, upset at the treatment of Muslims worldwide, were among those arrested.

The newspaper said they had trained at a camp north of Toronto and had plotted to attack CSIS’s downtown office near the CN Tower, among other targets.

Melisa Leclerc, a spokeswoman for the federal Public Safety Minister Stockwell Day, had no comment on the arrests.

In March 2004, Ottawa software developer Mohammad Momin Khawaja became the first Canadian charged under the country’s Anti-Terrorism Act for alleged activities in Ottawa and London. Khawaja was also named, but not charged, in British for playing a role in a foiled bomb plot. He is being held in an Ottawa detention center, awaiting trial.

The Canadian anti-terrorism law was passed swiftly following the Sept. 11 assaults, particularly after Osama bin-Laden’s named Canada one of five so-called Christian nations that should be targeted for acts of terror. The others, reaffirmed in 2004 by his al-Qaida network, were the United States, Britain, Spain and Australian, all of which have been victims of attacks.

The anti-terrorism law permits the government to brand individuals and organizations as terrorists and gives police the power to make preventive arrests of people suspected of planning a attack.

Though many view Canada as an unassuming neutral nation that has skirted attacks, it has suffered its share of aggression, including the 1985 Air India bombing, in which 329 people were killed, most of them Canadian citizens.

Intelligence officials believe at least 50 terror groups now have some presence in the North American nation and have long complained that the country’s immigration laws and border security are too weak to weed out potential terrorists.