VANCOUVER -- B.C.'s police complaint commissioner has ordered an investigation into the arrest of an innocent Indigenous man and his granddaughter at a Bank of Montreal branch.

Bella Bella resident Maxwell Johnson said it was heartbreaking to watch his 12-year-old granddaughter arrested while he was trying to help her open a bank account last month in Vancouver.

"It was so hard to see my granddaughter taken out of the bank and handcuffed. She will be scarred for life from this," Johnson said in an emotional Facebook post after the Dec. 20 incident.

"At the time I was just worried … now that I have had time to go over this it makes me so mad."

According to the Vancouver Police Department, a bank employee grew suspicious of the family during the transaction and decided to report them for possible fraud. Police acknowledged the responding officers placed Johnson and his granddaughter in handcuffs until they were able to confirm their identities and determine nothing illegal had taken place.

The incident prompted widespread outrage and led to multiple public complaints to the Office of the Police Complaint Commissioner. On Tuesday, the OPCC announced it has asked the Delta Police Department to conduct an external investigation into what happened.

"It is important that there be a thorough and independent investigation of this matter," Deputy Police Complaint Commissioner Andrea Spindler said in a statement Tuesday.

"The investigation will carefully examine and assess the circumstances of this incident including the legal authority to detain, arrest and use restraining devices such as handcuffs as well as any relevant questions of policy or training."

The OPCC said it will not be releasing any further information while the matter is under investigation.

In the meantime, a support rally for the family has been organized at the Britannia Community Centre in Vancouver on Tuesday afternoon. Johnson said he will unfortunately not be able to attend due to the weather.

Much of the public indignation has also been directed at the bank. National Chief Perry Bellegarde of the Assembly of First Nations called on BMO to "set better standards of their employees" in order to avoid something similar from happening in the future.

BMO has not responded to questions from CTV News about whether the employee involved is being disciplined or given additional training. The bank did address the incident on Twitter, stressing that it "does not reflect us at our best."

"We deeply regret this and unequivocally apologize to all," it reads. "We are reviewing what took place, how it was handled, and will use this as a learning opportunity. We understand the importance and seriousness of this situation at the highest levels at the bank."

The Vancouver Police Department acknowledged the situation was "upsetting and distressing" for Johnson and his granddaughter, and said officers already receive cultural competency training on an ongoing basis.

"The VPD are always looking at ways to be better at what we do and how to deal with each situation, especially sensitive issues and cultural differences," Const. Tania Visintin said in an email.

"This is a regrettable situation and our officers do have to deal with incidents which are sensitive in nature."

It's unclear why the Bank of Montreal employee felt it necessary to call police. Johnson said he was told there was an issue matching his Indian status card number with the one listed in the bank's computer system, but that he answered the employee's questions as best he could.