Both Bellegarde and Stewart highlighted their concerns with provisions in the bill that some say could lead to peaceful protests and other activist efforts being targeted as potential terrorism.
Last month, Bellegarde voiced those fears in an interview with CBC Radio's The House.
"It's really flawed legislation," he said.
He told host Evan Solomon that he believes First Nations people have an "obligation" to object to pipelines, mines and other development that could have an impact on their traditional territories, land and water.
"if people stand up and start using and exercising political activism to protect the lands and water, they'll be deemed terrorists, according to C-51," he said.
"And that's not the case."
Public Safety Minister Steven Blaney did his best to debunk those concerns during his appearance before the committee on Tuesday.
"The leader of the NDP has alleged that the legislation before us today means that legitimate dissent and protests would now be considered threats to Canadian security," he noted.
"These allegations are completely false, and, frankly, ridiculous"
The act, he continued, "clearly states that the definition of activity that undermines the security of Canada does not include lawful advocacy, protest, dissent or advocacy expression."
And while Privacy Commissioner Daniel Therrien may not have made the short list, some of his major concerns have already been brought to the committee table by other witnesses, including British Columbia Civil Liberties Association counsel Carmen Cheung and lawyer Paul Champ.
University of Calgary professor Barry Cooper also appeared during the morning session.
This evening, the committee will hear from law professors Craig Forcese and Kent Roach, who have been churning out extensive analysis on the bill since it was tabled earlier this year, as well as National Airlines Council of Canada executive director Marc-Andre Rourque and National Council of Muslims executive director Ihsaan Gardee.
On Tuesday, the committee heard from Blaney and Justice Minister Peter MacKay, as well as RCMP Commissioner Bob Paulson and CSIS head Michael Coulombe.
The full witness schedule for Thursday:
Morning session (8:45-10:45 a..m. ET)- British Columbia Civil Liberties Association counsel Carmen Cheung
- Greenpeace Canada executive director Joanna Kerr and energy campaign head Keith Stewart
- Former Security Information Review Committee (SIRC) chair Ron Atkey
- Assembly of First Nations National Chief Perry Bellegarde
- International Civil Liberties Monitoring Group lawyer Paul Champ
- University of Calgary political science professor Barry Cooper
Evening session (6:30-8:30 p.m. ET)- National Airlines Council of Canada executive director Marc-Andre O'Rourke
- University of Ottawa professor Craig Forcese and University of Toronto professor Kent Roach
- National Council of Muslims executive director Ihsaan Gardee
- Amnesty International Canada secretary general Alex Neve
- Carleton University professor Elliot Tepper