When the elder Mr. Trudeau declared an emergency nearly 52 years ago, he relied not on the law his son used Monday but on a predecessor, the War Measures Act, and his challenge was not civil unrest but terrorism. A group of Quebec separatists, who had conducted a bombing campaign in Montreal, had kidnapped Quebec’s deputy premier and a British diplomat. Pierre Laporte, the deputy premier, was later assassinated.On Monday, several national security experts praised the current prime minister’s decision.“The Emergencies Act was necessary in the face of the breakdown of law and order in parts of Canada and the economic and reputational costs that Canada suffered with some of its allies, particularly the United States,” said Wesley Wark, a national-security expert and a senior fellow at the Center for International Governance Innovation, a Canadian public policy group. “I expect some stepped-up law enforcement in the next couple of days.”Leah West, a professor who studies national security law at Carleton University, said she believed some of the restrictions on rights introduced by the law, like those on movement, are acceptable under Canada’s Charter of Rights and Freedoms. But she said the financial measures may be more problematic.Many Canadians, and Ottawa residents in particular, have shown increasing impatience over what they view as an anemic police response to the protests, which began as a truck convoy in the western province of British Columbia and reached the capital on Jan. 29. While there has been little physical violence, lives in the areas surrounding the Parliament have been disrupted and the police are investigating several complaints of hate crimes and harassment.Protesters, whose numbers swell on weekends, have desecrated the national war memorial, and legal violations like public drinking are widespread. An upscale shopping mall just blocks from Parliament is now into its third week of closure. Stores there have lost tens of millions of dollars in sales and about 1,500 workers lost wages.Peter Sloly, the police chief of Ottawa, has repeatedly said that his force, which had jurisdiction over the protests until the emergency declaration, is outnumbered and asked for upward of 1,800 more officers.But on Sunday, Bill Blair, the federal emergency preparedness minister and a former police chief of Toronto, said he found the lack of action by Ottawa’s force “inexplicable.”