One day after the police chief in Canada’s capital said his city was under “siege” by thousands of truckers and other protesters angry over government policies, the mayor on Sunday declared a state of emergency and called for outside help.

“We’re in the midst of a serious emergency, the most serious emergency our city has ever faced,” the mayor of Ottawa, Jim Watson, said in a television interview after declaring the emergency. “And we need to get moving much more quickly and much more proactively to bring order back to the streets.”

“Someone is going to get killed or seriously injured because of the irresponsible behavior of some of these people,” the mayor warned.

Across Canada this weekend, thousands of protesters took to the streets for the second week in a row, snarling traffic and disrupting business and residential neighborhoods. The truckers, whose cross-country convoy sparked the protest, paralyzed downtown Ottawa and the area around Parliament, parking their vehicles in intersections and across busy thoroughfares.

Protests took place in Ottawa, Toronto and Quebec City, while truck convoys of varying sizes congregated near provincial legislatures in Saskatchewan, Manitoba and British Columbia.

The protests were initially set off by vaccine mandates for truckers crossing the border from the United States, but have since expanded to draw thousands of other protesters from Canada’s political right unhappy over a mixed bag of issues, including the leadership of Prime Minister Justin Trudeau.

The protests are also fanning tensions between the right to freedom of speech and protest in Canada and the imperative by law enforcement to maintain public order.

Mr. Trudeau, who is isolating after testing positive for Covid-19 last week, has called the protesters a “small fringe minority,” and last week he lashed out at demonstrators in Ottawa for desecrating war memorials, wielding Nazi symbols, spreading disinformation and stealing food from the homeless.

He and his family left their downtown home last weekend, and his location has not been disclosed.

Mr. Trudeau has repeatedly said that 90 percent of Canada’s truckers are vaccinated, an assessment shared by the Canadian Trucking Alliance, a trade group that opposes the protest. Moreover, opinion polls have consistently shown strong support in Canada for public health measures to limit the spread of the coronavirus.

Nevertheless, the protesters have tapped into pandemic exhaustion, frustration and anger in Canada after months of lockdowns and truncated lives.

The atmosphere of the demonstrations this weekend was boisterous and by and large peaceful, even festive.

But in Ottawa, the Canadian capital, the authorities said they were overwhelmed. On Sunday, the mayor said the city was left with little choice. The protests have paralyzed the downtown core with traffic, noise and complaints of harassment, and some residents have said they are afraid to leave their homes.

“Declaring a state of emergency reflects the serious danger and threat to the safety and security of residents posed by the ongoing demonstrations,” Mr. Watson said in a statement.

The Ottawa police have also launched several criminal investigations into the desecration of monuments and other acts that occurred last weekend when the protests began.

Mr. Trudeau has ruled out using the military to disband the protests. But the police plan to try to choke off the truckers’ supply of diesel fuel to prevent them from constantly idling their engines and fouling the air.

“It’s an absolute disgrace when they’re bringing in bouncy castles and hot tubs and saunas,” Mr. Watson said in the TV interview. “Complete insensitivity to the people who are living through this terrible situation in the residential neighborhoods.”

One city councilor, Catherine McKenney, spoke last week of being deluged with complaints.

“I’m receiving hundreds — and I’m not exaggerating — hundreds of emails telling me: ‘I went out to get groceries, I got yelled at, I got harassed. I got followed down the street, I’m so afraid that I can’t go out,’” the councilor said.

In Ottawa, the authorities warned that the noisy and disruptive protests posed a real threat.

“This is a siege — it is something that is different in our democracy than I’ve ever experienced in my life,” Peter Sloly, chief of the Ottawa Police, said on Saturday. “We do not have sufficient resources to adequately and effectively address this situation” while tending to routine policing, he said.

Throughout the area, many businesses have been forced to shut their doors over the past week, resulting in tens of millions of dollars in lost sales. Those that have remained opened have struggled to enforce provincial mask rules.

Other cities were also buffeted by the protests over the weekend. In Toronto, Canada’s largest city and financial capital, dozens of cars, pickup trucks and heavy trucks on Saturday were parked along the city’s high-end shopping district downtown. Some of the protesters honked their horns, and shouts of “freedom” rang out.

And in Quebec City, dozens of tractor-trailer cabs were parked two deep for three blocks along one of the major arteries through the downtown area, adjacent to the provincial legislature. Thousands of people lined the sidewalks, cheering on truckers as they arrived or drove past.

Emmett Lindner contributed reporting.