Addressing the House of Commons just meters from the spot where the gunman, a reported convert to Islam, was shot dead on Wednesday, Harper said lawmakers would expedite new powers to counter the threat of radicals.
"The objective of these attacks was to instill fear and panic in our country," Harper said. "Canadians will not be intimidated. We will be vigilant, but we will not run scared. We will be prudent but we will not panic."
Harper pledged to speed up a plan already under way to bolster Canadian laws and police powers in the areas of "surveillance, detention and arrest."
Harper said the attack - following an incident on Monday when a convert to Islam ran over two Canadian soldiers with his car, killing one - would strengthen Canada's response to "terrorist organizations."
The attacks in Ottawa and Quebec took place as the Canadian government prepared to boost the powers of its spy agency, the Canadian Security Intelligence Service.
Public Safety Minister Stephen Blaney said last week the new legislation would let the agency track and investigate potential terrorists when they travel abroad and ultimately prosecute them.
At the start of parliament's session on Thursday, the guard credited with killing the gunman received a prolonged standing ovation, reopening debate in the House of Commons dressed in his usual ceremonial garb and struggling to maintain composure.
Harper and members of parliament stood in the legislature as Kevin Vickers, Canada's Sergeant-at-Arms, led the traditional parade that opens every session of the House of Commons.
While parliament resumed, tensions in Canada's capital remained high.
Police arrested a man at gunpoint just steps from the prime minister as Harper and his wife were laying a wreath at the National War Memorial to commemorate the killing of the soldier there, Cpl. Nathan Cirillo, 24.
Police, shouting and with guns drawn, surrounded a man and ordered him to the ground. Ottawa Police said the man was arrested for "disturbing the crime scene" at the war memorial. The man's intent was not immediately clear.
Harper himself was pulled back from the crime scene after he and his wife briefly lifted the crime scene tape and attempted to lay flowers, and then reversed themselves and laid their wreath outside the crime scene.
The tense moment was captured on camera and seen by throngs of people and politicians who had gathered at the war memorial.
Some vowed not to let the shooting mar the Canadian capital's open atmosphere, where government buildings are far more open to the public than in the neighboring United States.
"We have to be very introspective before we make any rash decisions," Peter MacKay, minister of justice, told reporters. "We're not talking about closing off the hill, we're talking about ways we can ensure greater security."
Police said on Thursday they were satisfied that only one person was involved in the attack.
Canadian police were investigating a man named Michael Zehaf-Bibeau as the possible suspect, said a source familiar with the matter. U.S. officials said they had been advised he was a convert to Islam.
The attacks on soldiers in Ottawa and near Montreal took place after Canada announced this month it would send six jets to take part in air strikes against Islamic State fighters who have taken over parts of Iraq and Syria.
A leading British Islamist preacher, Anjem Choudary, rejected assertions he had influenced either attack but warned that Western military involvement in the Middle East could inspire similar attacks. Monday's attacker followed Choudary on Twitter.
"People need to wake up to the reality that their own countries are being dragged into a war far away - the Syrian and Iraqi people are not occupying Britain or Canada - so they are seen as the aggressors," Choudary told Reuters.
Defense Minister Rob Nicholson said Canada's deployment to Iraq would go on unimpeded.
Tighter security was evident all over the sprawling parliamentary zone in downtown Ottawa. Armed Royal Canadian Mounted Police officers stood outside the door where the gunman rushed in on Wednesday.
The flag flying over Parliament's Center Block, where the gunman had burst in on Wednesday morning, was at half mast.
"There was only one gunman," said an RCMP officer who was guarding Parliament Hill early on Thursday.
He said in the confusion on Wednesday morning, witnesses saw things from different angles, suggesting the possibility of a second shooter but videos and further interviews showed this was not the case.
Bullet holes could be seen in the carpet just inside the front door and in the masonry in the hallway where he was shot.
In front of the war memorial, a crowd of about 100 people gathered at the police barricades in front of the war memorial early on Thursday.
"We're devastated that this has happened. I feel terrible for the individual that has lost his life, Cpl. Cirillo," said Stephen Miller, who was standing at the barricades holding a bouquet of flowers.
Asked if he was afraid for his safety being at the monument, Miller said, "Maybe a little but there is no courage without fear."