Misbahuddin Ahmed, left, and Hiva Alizadeh made a brief court appearance in Ottawa on Thursday on charges related to what the RCMP say was a domestic terrorist plot. (Sarah Wallace/CBC)

Three Ontario men accused of taking part in a domestic terrorist plot and possessing plans and materials to create makeshift bombs had allegedly selected specific targets in Canada, sources told CBC News.

The suspects are alleged to have discussed attacks on specific government buildings and city public transit systems, security sources told CBC News.

But none of the targets was in the United States, sources said.

Misbahuddin Ahmed, 26, and Hiva Alizadeh, 30, both of Ottawa, and 28-year-old Khurram Sher, of London, Ont., have all been charged with conspiracy to knowingly facilitate a terrorist activity.

Alizadeh is also charged with being in possession of an explosive substance with intent to harm and providing property or financial services for the benefit of a terrorist group.

Ahmed and Alizadeh were arrested Wednesday in Ottawa. Sher was arrested Thursday in London in southwestern Ontario.

The three men were involved in a conspiracy to commit "a violent terrorism attack," said RCMP Chief Supt. Serge Therriault.

Therriault alleged at a news conference in Ottawa Thursday that the men conspired with three other men, whom they named as James Lara, Rizgar Alizadeh and Zakaria Mamosta (who Reuters reported are not in Canada), and other unnamed individuals in Canada, Iran, Afghanistan, Pakistan and Dubai to commit terrorism-related offences.

Khurram Sher auditioned for the 2008 edition of popular reality show, Canadian Idol, in Montreal. (YouTube)

He said the RCMP investigation, dubbed Project Samossa, found evidence that one member of the group had been trained to construct electronic and explosive devices.

During their investigation, Therriault said, police seized more than 50 electronic circuit boards they say were designed specifically to remotely detonate improvised explosive devices (IEDs).

He said they also seized a vast quantity of terrorist literature, videos and manuals.

"This group posed a real and serious threat to the citizens of the National Capital Region and Canada's national security," he said.

"Our criminal investigation and arrests prevented the assembly of any bombs and the terrorist attack or attacks from being carried out."

Investigators conducted "extensive surveillance" of the suspects during the yearlong investigation, Therriault said.

According to a document seen by CBC News, police made surveillance recordings at the Ottawa townhouse rented by Misbahuddin Ahmed.

People were allegedly overheard praying together and talking about Canada's anti-terrorism law, the structure of terror cells and the so-called Toronto 18. But a security source tells CBC News that the men mostly communicated over the internet.

Investigators said they have reason to believe Hiva Alizadeh is a member of, and in contact with, a terrorist group with links to the conflict in Afghanistan but declined to name the group.

Therriault said investigators have grounds to believe that the three men are "part of a domestic terrorist group" operating in Canada.

He said part of the decision to make the arrests now was to prevent one of the suspects from providing financial support to terrorist counterparts abroad.

"Canada is not immune to terrorism," Public Safety Minister Vic Toews said from Winnipeg on Thursday. "We are not immune from international or home-grown radicalization."

He also said authorities need co-operation from various community groups and called on Canadians to "stand up and be vigilant against the terrorist threat against our nation."

Speaking from Tuktoyaktuk, N.W.T., Prime Minister Stephen Harper said that the case "does serve to remind us that Canada does face some very real threats in the troubled world in which we live."

RCMP investigators remove evidence boxes from Sher's home in London, Ont., on Thursday. (Dave Chidley/Canadian Press)

He said the case is a police matter but noted that the government will work with police and security agencies to ensure they have the resources they need.

Ahmed and Alizadeh made a brief court appearance in Ottawa on Thursday and were remanded into custody until their next appearance on Sept. 1.

Ahmed worked as an X-ray technician at an Ottawa hospital.

According to court documents, Alizadeh lived in Manitoba for some time and attended Red River College in Winnipeg before coming to Ontario.

The college confirmed that a man named Hiva Alizadeh studied English as a second language there in 2003-04 and electrical engineering in 2008-09.

The synopsis of the case said Alizadeh had been under surveillance for some time and had met with other accused at his home and other locales. Police allege he was recorded discussing the Toronto 18 terrorism case, Canada's system of security certificates, which allow permanent residents and foreign nationals to be detained and deported based on secret evidence, and the structure of terrorist cells.

Alizadeh also allegedly went to Iran for several months, according to court documents.

Police allege Sher is a McGill medical graduate who travelled to Pakistan in 2006 to help with earthquake relief and also auditioned for the Canadian Idol singing competition in the past.

All three men are Canadian citizens.