Police said the suspects planned "al-Qaeda-inspired" attacks
Five other youths have also been charged, following an investigation involving more than 400 officers.
Police seized bomb-making materials in a series of raids in Toronto, including three metric tons of ammonium nitrate.
Officials said the group "posed a real and serious threat" with "the capacity and intent to carry out these attacks".
Fifteen of the suspects appeared in a heavily guarded courtroom in Brampton, a Toronto suburb, on Saturday.
Some family members sobbed during the hearing while others attempted to speak or wave to the detainees, Reuters news agency reports.
A list of the names and addresses of the 12 adults, which was released after the arrests were made, indicates that they are all resident in Toronto or the surrounding province of Ontario.
'Enough for three Oklahomas'
Ammonium nitrate is a commonly used fertiliser which has also been used to make bombs.
One guy was doing some criminal activity, selling guns for money
Aly Hindy
Imam at a Toronto mosque who knows some of the accused
"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," said assistant commissioner Mike McDonell of the Royal Canadian Mounted Police.
"Our investigation and arrests prevented the assembly of any bombs and the attacks from being carried out."
Southern Ontario is one of the country's main economic and business centres.
The Mounties would not name any of the suspected bombing targets.
Prime Minister Stephen Harper said Canada had been targeted because of its way of life and "was not sheltered from the terrorist threat".
"Today, Canada's security and intelligence measures worked. Canada's new government will pursue its efforts to ensure the national security of all Canadians," he added.
'Violent ideology'
Officials showed what they said was evidence of bomb-making materials, a computer hard drive, camouflage uniforms and what appeared to be a door with bullet holes in it.
Police seized an array of bomb-making materials
The Mounties and other government security agencies, including intelligence and border security, have been conducting a lengthy investigation, the largest of its kind in Canada.
Police said those arrested on Friday were all Canadian residents "of different origins", most of them citizens - some were students, some employed, others unemployed.
Most of the 12 adults, whose ages range from 19 to 43, have Arabic names but police say no one community should be singled out.
Muslim leaders in Toronto have condemned the planned attack and said extremist messages had been preached in some area mosques in recent years.
The suspects appear to have "chosen a violent ideology inspired by al-Qaeda", said Luc Portelance, assistant director of operations for the Canadian Security Intelligence Service (CSIS), Canada's spy agency.
Aly Hindy, an imam at a Toronto mosque, said he knew most of the accused and believed one or two were involved in crime but not terrorism.
"One guy was doing some criminal activity, selling guns for money," he was quoted as saying by Reuters news agency outside the courtroom.
More arrests are said to be possible.
Seventeen people, including five minors, have been arrested in the Toronto area on terrorism-related charges.  Over 400 officers, drawn from the Royal Canadian Mounted Police (RCMP), the Canadian Security Intelligence Service (CSIS), and the Toronto, Peel, and Durham police departments raided a dozen locations throughout the greater Toronto area.  More arrests are expected overnight.

Anonymous sources have told Canadian Press that those arrested are suspected to have been inspired by al Qaeda, and that they had plans to detonate explosives somewhere in Ontario.  According to BBC News, three tonnes of ammonium nitrate, a common fertilizer that can be used as an explosive, have been seized.  RCMP assistant commissioner Mike McDonnell stated, "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.”  In order to protect the investigation, he was unable to provide any information as to the potential targets, but did state that the Toronto Transit Commission was not targeted. 

The suspects are currently being held under extremely heavy security at a police station in Pickering, just east of Toronto.  According to RCMP spokeswoman CPL. Michele Paradis, “The investigation is ongoing.”  The suspects are expected to appear in court today, in Brampton.
They have all been charged under  Section 83 of the '' Criminal Code of Canada''. 

The father of one of the suspects, Mohammed Abdelhaleen, told reporters: "I'm shocked. It's crazy - it's just crazy. It has no meaning whatsoever."