OTTAWA, Ontario — A showdown appeared to be shaping up in Ottawa's nearly three-week siege by truckers protesting the country's COVID-19 restrictions as police in the capital warned drivers on Wednesday to leave immediately or risk arrest.

The big rigs parked outside Parliament represented the movement's last stronghold after demonstrators abandoned their sole remaining truck blockade along the US border. With that, all border crossings were open for the first time in more than two weeks of unrest, centering attention on the capital, where drivers defiantly ripped up warnings telling them to go home. Authorities in yellow "police liaison" vests went from rig to rig, knocking on the doors and handing truckers leaflets informing them they could be prosecuted, lose their licenses and see their vehicles seized under Canada's Emergencies Act. Police also began ticketing vehicles.

One protester shouted, "I will never go home!" Some threw the warning into a toilet put out on the street. Protesters sat in their trucks and honked their horns in a chorus that echoed loudly downtown.

There was no immediate word from police on when or if they might move in to clear the hundreds trucks by force. But protest leaders braced for action on Wednesday.

"If it means that I need to go to prison, if I need to be fined in order to allow freedom to be restored in this country — millions of people have given far more for their freedom," said David Paisley, who traveled to Ottawa with a friend who is a truck driver.

The warnings came two days after Prime Minister Justin Trudeau invoked the emergency law to try to break the protests.

"It's not for politicians to tell police when and how to do things. What we have done with the emergency act is to make sure the police have the necessary tools," Trudeau said Wednesday. "It's something that I, like all residents of Ottawa, hope to happen soon." The crisis has become one of the most serious tests yet for Trudeau, the boyish-looking 50-year-old who has long channeled the star power — if not quite the political heft — of his father, Pierre Trudeau, who was prime minister a generation ago.

Some lawmakers are faulting the younger Trudeau for not moving more decisively against the protests, while others are accusing him of going too far in assuming emergency powers.

Since late January, protesters in trucks and other vehicles have jammed the streets of the capital and obstructed border crossings. The demonstrations by the self-styled Freedom Convoy initially focused on Canada's vaccine requirement for truckers entering the country, but soon morphed into a broad attack on COVID-19 precautions and Trudeau himself.

On Wednesday, protesters who had stopped traffic and trade for a week along the US border at Emerson, Manitoba, opposite North Dakota, pulled away in tractors and trucks without any arrests.

The protests have drawn support from right-wing extremists and have been cheered on and received donations from conservatives in the US, triggering complaints in some quarters about America and its pandemic politics being a bad influence on Canada.

Daniel Bulford, a protest leader who described himself as a former officer with the Royal Canadian Mounted Police and a recent member of Trudeau's security detail, accused the Trudeau government of resorting to "extreme and authoritarian" measures to quell the demonstrations.

Organizers of protests encouraged supporters to come to the capital to make it difficult for police to clear them out. But the nation's top safety official warned them to stay away or face legal consequences too.

Meanwhile, the premiers of two Canadian provinces and 16 US governors sent a letter to Trudeau and US President Joe Biden calling on them to end their nations' vaccine mandates for truckers crossing the border.

Over the past weeks, authorities hesitated to move against many of the protesters around the country, citing in some cases a lack of manpower and fears of violence. But the bumper-to-bumper occupation has infuriated many Ottawa residents, who have complained of being harassed and intimidated on the clogged streets. The rising frustration cost Ottawa Police Chief Peter Sloly his job this week.

As of Tuesday, Ottawa officials said 360 vehicles remained involved in the blockade in the city's core, down from a high of roughly 4,000. "They don't want to give this up because this is their last stand, their last main hub," said Michael Kempa, a criminology professor at the University of Ottawa.

An Ottawa child welfare agency advised parents at the demonstration to arrange for someone to take care of their children in the event of a police crackdown. Some protesters had their youngsters with them.

Police in the capital appeared to be following the playbook that authorities used over the weekend to break the blockade at the economically vital Ambassador Bridge connecting Windsor, Ontario, to Detroit. Police there handed out leaflets informing protesters they risked arrest. After many of those demonstrators left, police moved in and arrested dozens who remained. The blockade had disrupted the flow of goods between the two countries and forced the auto industry on both sides to curtail production.

Stephanie Carvin, who once worked for Canada's domestic intelligence service and teaches national security at Carleton University in Ottawa, said police in the capital face a tricky situation. Some of the protesters are extremists, and police run the risk of violence if they try to disperse or arrest them, she said.

"The last thing we want is any kind of propaganda that can really feed the flames of this movement for years to come," Carvin said.

Gillies reported from Toronto. Associated Press writers Robert Bumsted in Ottawa and Andrew Selsky in Salem, Oregon, contributed to this report.