MONTREAL -- Two Canadian women detained in Mexico City in connection with a recent firebomb attack are still waiting to learn whether they will face criminal charges -- or whether they will be set free.

Mexican authorities say they arrested Canadians Amelie Pelletier and Fallon Rouiller Poisson after Molotov cocktails struck a federal office and a nearby car dealership earlier this week in the capital.

Mexico's federal attorney-general's office tells The Canadian Press that local authorities are currently considering the firebombing an act of vandalism.

But Jose Luis Manjarrez says the ongoing investigation could lead to serious criminal charges against the Canadians, who were apprehended along with a Mexican man.

Manjarrez says investigators are still trying to determine what kind of role they may have played in an attack that caused no injuries, but damaged several vehicles at the dealership.

He says police are permitted to detain the women until Thursday night -- when they will either be charged or released.

A Mexico City prosecutor has told local media that police allege one of the Canadians was seen discarding a backpack that contained a number of homemade explosive devices.

On Thursday in Montreal, dozens of people showed their support for Pelletier and Poisson by holding a vigil outside the front entrance of a downtown building that houses the Mexican Consulate.

Organizers of the vigil declined to answer questions from reporters, but a statement was read aloud to the group by well-known activist Jaggi Singh.

The statement calls the Mexican justice system "repressive," "authoritarian" and one that considers detainees guilty until proven innocent.

It also says the group has concerns about the detention conditions of their comrades and how it has been unable to make direct contact with them.

"Free our comrades," the participants chanted as they huddled on the sidewalk. Group members also unfurled a banner with a message, partially Spanish and partially French: "Not guilty, Not innocent, SOLIDARITY."

Poisson's loved ones have also expressed concern following her arrest in Mexico City.

Her mother, Line Rouiller, told The Canadian Press on Wednesday she's concerned about the well-being of her 20-year-old daughter, who grew up in the Montreal area.

Those who know Poisson, a recent junior-college graduate, have said she was an active participant in the 2012 student movement that led to nightly, large-scale demonstrations two years ago in Quebec -- a crisis dubbed the Maple Spring.

She was also described as someone with strong political convictions.

Manjarrez said the Molotov cocktails struck the facade of a building that contains the offices of the Department of Communications and Transportation. The motive was not immediately clear.

"For now, for the authorities, it is considered an act of vandalism and not a terrorist act," Manjarrez said Thursday in a phone interview from Mexico.

"It's an act of provocation by people who are demonstrating their disagreements by using radical means.

"But to be considered a terrorist act, it must correspond to a precise legal definition and that will be up to a judge to determine after the evidence has been examined."

Canada's Foreign Affairs Department has confirmed that officials have been in contact with Mexican authorities on behalf of two Canadian citizens detained in Mexico, but said that it could not release any more information.

"What I can say is consular services are being provided as required," department spokesman John Babcock wrote Thursday in an email.

"To protect the privacy of the individuals concerned, further details on this case cannot be released."