Canadian police massed in the capital Thursday, readying to clear a trucker-led protest that has choked Ottawa's streets for three weeks and provoked the government to call on rarely used emergency powers.

With barricades going up and a heavy police presence forming in the area where hundreds of big rigs remained parked, Prime Minister Justin Trudeau was in Parliament nearby defending his decision to invoke the Emergencies Act for only the second time ever in peacetime.

"Illegal blockades and occupations are not peaceful protests," Trudeau told the House of Commons, adding: "They have to stop."

In response to critics, he said the act was not being used to call in the military against the protesters, and denied restricting freedom of expression.

The objective was simply to "deal with the current threat and to get the situation fully under control," he said.

Throughout the day police officers could be seen filing into the parliamentary precinct in Ottawa where the truckers appeared to dig in.

Public Safety Minister Marco Mendicino said the situation was "precarious."

The demonstrators had been given an ultimatum late Wednesday by the capital city's interim police chief to "leave the area now," or risk arrest and truck seizures.

In a statement, Chief Steve Bell said "a methodical and well-resourced plan" would be carried out over the coming days "to take back the entirety of the downtown core and every occupied space."

"Some of the techniques we are lawfully able and prepared to use are not what we are used to seeing in Ottawa," he said. "But we are prepared to use them... to restore order."

Truckers responded by blaring horns through the night and into Thursday. Waving Canadian flags on the ends of hockey sticks, they also chanted "freedom!"

One of the protest leaders, Tamara Lich posted a tearful video to say she was expecting to be arrested. "I think it's inevitable at this point... I'm okay with that," she said.

She also called on supporters to flood the capital, saying truckers already in place "are gonna stay and fight for your freedom."

"If you can come to Ottawa and stand with us, that would be fantastic," she said. "I want you to keep fighting the good fight."

 Potential for 'terrorist attacks'

Facing growing pressure to dislodge the protesters, Trudeau this week invoked emergency powers to end their occupying Ottawa and until recently blocking border crossings to the United States -- which Mendicino said has cost the economy billions of dollars.

Such a move has only been used once before, in 1970 by his father, former prime minister Pierre Trudeau, to crack down on Quebec separatists who'd kidnapped two officials and set off bombs in Montreal.

"It's time for people to go home," Deputy Prime Minister Chrystia Freeland told a news conference, warning truckers that the penalties for staying "will bite."

"Let me also be clear that we will have zero tolerance for the establishment of new blockades or occupations," she said after a convoy was stopped late Wednesday from re-occupying the Ambassador Bridge -- a key trade route between Windsor, Ontario and Detroit, Michigan.

In documents filed to the Commons, the government laid out its rational for invoking the emergency powers, saying the trucker convoy has created a critical and urgent situation that cannot be dealt with under any other Canadian laws.

It cited "a risk of serious violence and the potential for lone actor attackers to conduct terrorism attacks."

In a letter to provincial premiers, Trudeau decried the protests as "a threat to our democracy."

"It is affecting Canada's reputation internationally, hurting trade and commerce, and undermining confidence and trust in our institutions," he added.

The so-called "Freedom Convoy" started with truckers protesting against mandatory Covid vaccines to cross the US border, but its demands have since grown to include an end to all pandemic health rules and, for many, a wider anti-establishment agenda.

At its peak, the movement also included blockades of a half-dozen border crossings that have now been cleared.

Police this week arrested dozens of protesters, including four people charged with conspiracy to murder police officers at a checkpoint between Coutts, Alberta and Sweet Grass, Montana.

They also seized dozens of vehicles, as well as a cache of weapons that included rifles, handguns, body armor and ammunition.

Freeland said bank accounts of protesters and their backers have been frozen "and more accounts will be frozen."

Authorities have also moved to choke off crowdfunding and cryptocurrency transactions supporting the protesters, she said.