Harper was speaking publicly for the first time since gunmen stormed the offices of the satirical newspaper Charlie Hebdo on Wednesday, killing 12 people and setting off a massive hunt for suspects.
"When a trio of hooded men struck at some of our most cherished democratic principles — freedom of expression, freedom of the press — they assaulted democracy everywhere," said Harper, who was in the Vancouver area for an announcement.
The gunmen killed eight journalists, two police officers, a maintenance worker and a visitor. Police in France were searching for two brothers, including one with a previous terrorism conviction, while a third suspect surrendered after hearing his name linked to the case.
Charlie Hebdo has a long history viciously lampooning political figures and religions. Its depictions of Islam and the Prophet Muhammad prompted threats against the newspaper and its staff.
The attack prompted rallies in a number of countries, including Canada, that saw attendees hold up pens to support the newspaper and condemn the attack.
"Yesterday, hundreds of thousands of people around the world, and in cities across this country, openly demonstrated that we will not be intimated by jihadist terrorists," Harper said.
"Today, I know all Canadians ... stand together with the people of France." our great friends and allies."
Witnesses said the attackers claimed allegiance to al-Qaida in Yemen and an unnamed French official told The Associated Press the suspects were linked to a Yemeni terrorist network.
Harper drew the connection between the Paris attack and what he described as the "international jihadist movement," including the Islamic State, also known as ISIL.
"They have declared war on anybody who does not think and act exactly as they wish they would think and act," said Harper.
"They have declared war and are already executing it on a massive scale on a whole range of countries with which they are in contact, and they have declared war on any country, like ourselves, that values freedom, openness and tolerance. We may not like this and wish it would go away, but it is not going to go away."
Harper said any doubts about the threat posed to Canada should have been washed away on Oct. 22 of last year, when a gunmen with jihadist sympathies killed a Canadian soldier at the National War Memorial before storming into Parliament, where he was shot dead. That attack came days after another man who espoused jihadist ideology used his car to kill a soldier in Quebec before he was fatally shot himself.
Harper stressed that even those attacks didn't mark Canada's first brush with terrorism.
He pointed to the so-called Toronto 18 case, in which a group of young men were arrested in 2006 and accused of plotting to bomb several high-profile targets; the alleged 2013 plot to bomb a Via Rail passenger train; and an alleged plot on Canada Day of 2013 to bomb the B.C. legislature in Victoria.
While Harper urged Canadians to be vigilant about the possible threat of terrorism, he urged them not to let it take over their lives.
"At the same time, we also encourage people to go about their lives and to exercise our rights and freedoms and our openness as a society as loudly and as clearly as we can," he said.
"Because that is the best way of defeating what is ultimately a movement of hatred and intolerance."
— Follow @ByJamesKeller on Twitter
With files from The Associated Press
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