The prime minister tied Wednesday's extraordinary events — the killing of an honour guard at the National War Memorial and a subsequent shootout inside Parliament's Centre Block — to an incident two days earlier in Quebec, where one soldier was killed and another injured when they were run down by a car driven by a man with declared sympathies for Islamic extremists.
Harper referred to the hit-and-run incident as "ISIL-inspired," a reference to the extremist Islamic State in Iraq and the Levant, against whom Canada has joined a U.S.-led bombing campaign in Iraq. The driver was killed by police.
The Parliament Hill gunman was himself shot dead by the sergeant-at-arms of the House of Commons.
"Fellow Canadians, in the days to come we will learn more about the terrorist and any accomplices he may have had," Harper said in a live televised statement.
"But this week's events are a grim reminder that Canada is not immune to the types of terrorist attacks we have seen elsewhere around the world."
Attacks on Canadian security personnel and governing institutions are, "by their very nature, attacks on our country, on our values, on our society, on us Canadians as a free and democratic people who embrace human dignity for all," he said.
But Canada will not be intimidated, Harper declared.
"In fact, this will lead us to strengthen our resolve and redouble our efforts, and those of our national security agencies, to take all necessary steps to identify and counter threats and keep Canada safe here at home — just as it will lead us to strengthen our resolve and redouble our efforts to work with our allies around the world and fight against the terrorist organizations who brutalize those in other countries with the hope of bringing their savagery to our shores.
"They will have no safe haven."
Harper offered the country's condolences to the family of Cpl. Nathan Cirillo, a 24-year-old reservist who was shot twice at point-blank range at the war memorial near Parliament Hill.
Cirillo was "murdered in cold blood" at a "sacred place that pays tribute to those who gave their lives so that we can live in a free, democratic and safe society," he said.
NDP Leader Tom Mulcair echoed Harper in expressing his revulsion for what he called "a cowardly attack designed to strike at the heart of our democracy, the heart of who we are."
But he appealed to Canadians not to give in to feelings of vengeance.
"We woke up this morning in a country blessed by love, diversity and peace," Mulcair said in a televised statement immediately following Harper's.
"And tomorrow we will do the same. These acts were driven by hatred but also designed to drive us to hate. They will not. We will stand up and we will stand together."
Similarly, Liberal Leader Justin Trudeau called the "brutal and heartless" acts of violence "unforgivable" and said any other accomplices must be caught and punished "to the full force of the law."
But he also urged Canadians not to let anger or fear change the country's values, describing Canada as an open, welcoming, fair, democratic, peace-loving country.
"Criminals cannot and will not dictate to us how we act as a nation, how we govern ourselves and how we treat each other ... and they do not get to decide how we use our shared public places," Trudeau said.
Of the three leaders, Trudeau was the only one to directly address Muslim Canadians.
"To our friends and fellow citizens in the Muslim community, Canadians know that acts such as these committed in the name of Islam are an aberration of your faith," he said.
"Continued mutual co-operation and respect will help prevent the influence of distorted, ideological propaganda posing as religion. We will walk forward together, not apart."
Conservative and NDP MPs had been gathered for their weekly caucus meetings Wednesday morning when the shots erupted in the Hall of Honour, which separates the caucus rooms of the two parties.
Harper, who had been addressing his MPs at the time, was whisked to safety at an undisclosed location.
Mulcair and his MPs barricaded the doors of their caucus room with chairs and tables and hid under tables before being escorted by security officials out of Centre Block. An aide said Mulcair insisted on staying with his MPs, most of whom were led through a tunnel to the adjacent East Block, where they remained under lockdown until the evening.
Trudeau and some aides had just emerged from his fifth-floor office and were on their way to the Liberal caucus meeting in the basement of Centre Block when the shootout erupted. They were told to return to the office and lock the doors.
Later in the day, as security officials conducted an office-by-office search, Trudeau and his staff were relocated to the fifth-floor cafeteria, where they remained under lockdown until after 8 p.m.
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