The Canadian prime minister, Justin Trudeau, has invoked legislation that gives his government sweeping powers to fight a growing number of “illegal and dangerous” blockades across the country.

The first prime minister to invoke the Emergencies Act, Trudeau said the measures would be time-limited and only apply to specific geographic regions. “We are not preventing the right of people to protest legally,” he said, adding that the military would not be deployed. “The act is to be used sparingly and as a last resort.”

Hours before Trudeau’s announcement, the Royal Canadian Mounted Police in Alberta announced that they had seized a truck full of firearms at blockade near the US border.

The Emergencies Act, which goes into effect for a month, allows the federal government to bar people from gathering in certain locations. It could also allow officials to conscript the use of private tow trucks. The deputy prime minister, Chrystia Freeland, said the Act would bar the use of crowdfunding websites for illegal activities and punish companies whose trucks were being used in the protests.

The recent hack of the crowdfunding site GiveSendGo revealed substantial financial support from outside Canada, which Freeland called an attack on democracy. “This is about following the money. This is about putting an end to these illegal blockades. Consider yourself warned,” she said.

While both the city of Ottawa and the province of Ontario have declared states of emergency, the use of the Emergencies Act is the most forceful use of government power so far in fighting the blockades.

The act replaced the War Measures Act in 1988 but is more limited in scope. The revised act requires parliamentary oversight and all measures must comply with the country’s Charter of Rights and Freedoms.

As the protests cripple trade routes and paralyse city streets, Trudeau said border officials were already turning away Americans trying to enter the country to join the demonstrations.

The prime minister’s unprecedented move comes amid growing worry from federal officials that protesters are increasingly unwilling to leave the blockades, as well as the rising prospect of violence.

The RCMP said on Monday that 11 people were arrested and detained and police seized 13 long guns, handguns, multiple sets of body armour, a machete, a “large quantity” of ammunition, and high-capacity magazines. For more than two weeks, protesters have blocked the border crossing near the town of Coutts, Alberta.

In a statement, police said they recently “became aware of a small organised group within the larger Coutts protest” that had access to weapons and “was said to have a willingness to use force against police” if any attempts were made to “disrupt” the blockade.

The Alberta premier, Jason Kenney, said the arrests and seizure of firearms “underscores the severity” of the situation.

In Ottawa, dozens of semi-trucks remained idling in front of the prime minister’s office – the third week of an occupation by protesters from across the country. A small crowd, including a number of children, braved frigid temperatures in the nation’s capital to protest public health measures and were unfazed by news of the Emergencies Act.

While elements of the demonstration had a festive feel, including volunteers handing out food and music blasting from a stage, other scenes evoked the dug-in mentality of the convoy. Groups guarded jerry-cans of fuel stashed at street corners amid the hum of generators in camper vans.

On Monday, an Ontario judge granted an injunction to enforce noise and idling bylaws related to the protests in Ottawa. The injunction effectively gives police a new tool for charging protesters in violation of a court order that bars fireworks, truck horns and idling.

Ottawa’s mayor tweeted that a number of trucks had started moving out of residential areas as part of an agreement with protest leaders, but cautioned that it could take days for many of the trucks to leave, calling it a “complex” operation.

But at a press conference, organisers of the convoy said protesters would remain until mandates were removed. Tamara Lich said Trudeau’s use of the Emergencies Act was “extreme”, but that the prospect of increased federal powers to end the blockades would deter protesters.

“No matter what you do, we will hold the line,” said Lich. “There are no threats that will frighten us. We will hold the line.”