Prime Minister Justin Trudeau said the government had been in the process of introducing the ban when its agenda was overturned by the pandemic.

OTTAWA — Nearly two weeks after the deadliest mass shooting in Canada’s history, Prime Minister Justin Trudeau on Friday introduced an immediate ban on what he described as “military-style assault weapons.”

“These weapons were designed for one purpose and one purpose only: to kill the largest number of people in the shortest amount of time,” Mr. Trudeau said. “There is no use and no place for such weapons in Canada.”

The ban means that Canadians will no longer be able to own rifles like the AR-15, the military-style weapon used in several mass shootings in the United States including those in Newtown, Conn.; Orlando, Fla.; and Parkland, Fla.

By introducing the ban, Mr. Trudeau partly fulfills a gun control promise he made during last year’s federal elections. He said the government had been in the process of introducing an assault weapons ban when its agenda was overturned by the coronavirus pandemic.

In making the announcement, Mr. Trudeau noted several gun killings and repeatedly cited the shooting rampage in rural Nova Scotia that left 23 people dead, including the gunman.

The gunman’s arsenal included two models banned on Friday, said Bill Blair, the country’s public safety minister. 

The killer did not have a firearms license and many of his guns and rifles had been smuggled into Canada from the United States, according to the Royal Canadian Mounted Police, highlighting one difficulty Canada may face in enforcing the new measure. The U.S. federal government has not barred assault weapons since a previous ban expired in 2004.

The swift response by Mr. Trudeau to the killings in Nova Scotia stands in contrast to that of officials in the United States, where repeated efforts to renew the now-lapsed assault weapons banhave failed.

The Canadian government has drawn up a list of about 1,500 gun models covered by the new ban. It estimates that about 100,000 such semiautomatic rifles are now legally owned by Canadians.

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Mr. Trudeau said the government will introduce legislation to buy back the rifles, another part of his campaign promise, at a future date. Until then, owners have been given two years to keep their rifles although they can no longer use them, trade them or sell them except to buyers outside Canada with a permit. Gun shops can return any of the weapons they now have in stock to manufacturers.

While handguns and automatic weapons are tightly restricted in Canada, most rifles and shotguns have been more loosely regulated. The previous Conservative government shut down a registry for such weapons that had been set up after a man gunned down 14 young women and injured 13 others in 1989 at the École Polytechnique engineering school in Montreal.

That database was beset by technical problems and was deeply unpopular in rural areas. Mr. Trudeau has resisted calls from gun control groups to revive it.

Mr. Trudeau said on Friday that his planned legislation will also include a measure that will allow cities to ban handguns within their boundaries, another of his campaign pledges.

Andrew Scheer, the leader of the Conservative Party, repeated his longstanding opposition to any ban and buyback of military-style weapons, noting that many mass killers, including Gabriel Wortman in Nova Scotia, and other criminals use illegal firearms brought in from the United States.

“It’s easy but lazy government to ask the people who follow all the rules to follow more rules,” Mr. Scheer told the Canadian Broadcasting Corporation. He also criticized Mr. Trudeau for introducing the measure through a cabinet order while Parliament is not meeting in normal sessions because of the coronavirus pandemic.

Wendy Cukier, the president of the Coalition for Gun Control, said that most mass shootings in Canada have involved legally owned rifles and said there’s evidence that the availability of military-style weapons may make such killings more likely.

“Most mass shooters are law abiding until they are not,” she said.

What motivated the 13.5-hour killing spree in Nova Scotia by Mr. Wortman, a denture fitter, remains unknown. It started in the tiny summer community of Portapique when Mr. Wortman assaulted his partner and tied her up. She escaped and he began shooting people inside and outside of their homes while he also set fire to several buildings, including some of his own properties.

After the police arrived shortly before midnight on April 18, they found two replica Royal Canadian Mounted Police cruisers registered to Mr. Wortman on fire and located a third at his full-time residence in Halifax. That led the police to believe, they said, that he may have committed suicide and was in one of the burning buildings.

But after hiding in the woods all night, Mr. Wortman’s partner told police that he was traveling in a fourth replica police car that did not have license plates. Investigators subsequently discovered that he had eluded them by driving through a farm field and then hiding in another town where he resumed his killing spree in the morning.

He was eventually shot and killed after pulling into a gas station while driving a car belonging to one of the victims.

Ms. Cukier acknowledged that the government will have to continually update its list to prevent manufacturers from circumventing the ban by modifying current models and reintroducing them as new weapons. Her group, she said, will recommend that future legislation focus more on a system in which gunmakers must get approval to sell specific weapons rather than on steps to ban the weapons.

And while her group generally takes stances that oppose those of the Conservatives, she agreed that more must be done about smuggled weapons.

“There are a lot of things that have to happen,” she said. “Most Canadians don’t know the extent to which our laws have been eroded.”

Alan Drummond, who has long pushed for more gun controls through the Canadian Association of Emergency Physicians, praised Mr. Trudeau and members of his cabinet for their unequivocal statements about the need to ban assault weapons.

“What struck me was the absolute clarity and conviction,” he said.