Six people, including two minors, are facing charges after two off-duty Montreal police officers were attacked in the city's downtown early Saturday morning.

The two officers, who are brothers, were walking along St-Denis Street when a group sitting on a nearby patio recognized them as members of the police force. They were not wearing their uniforms.

"They were walking to go have a bite and someone recognized them on a terrasse, yelled out, 'civilian police, civilian police,' and the next thing they knew they were being circled and punched and thrown to the ground," Insp. Marie-Claude Dandenault told reporters Tuesday.

Dandenault said both officers work in the area and those involved in the attack are known to police. But she said they had yet to identify a specific motive in the attack.

Witnesses filmed the incident and footage was posted on social media.

Dandenault said the investigation is ongoing and that more people could be arrested.

She said the officers are on leave and are recovering at their homes after being treated in hospital. One suffered a minor concussion and both are bruised.

"We are deeply affected by this. We find this to be totally unacceptable," Dandenault said.

Four of the alleged attackers, who are all in their 20s, have already appeared at the Montreal courthouse to face charges of assaulting a peace officer and intimidating someone associated with the justice system.

Among them is 20-year-old Mélina Geoffroy. She pleaded guilty on Tuesday to assault causing bodily harm. The other charges against her have been dropped. She is collaborating with police. Her sentence hearing is expected to be held next week.

Two other people, both minors, are expected to appear in court later today.

Under Canada's Criminal Code, assaulting a peace officer is an indictable offence and can come with a prison sentence of up to five years.

A representative from the Montreal Police Brotherhood said that the officers have the support of their union and that they will follow the case closely.