Troops across the country were told to stop wearing their uniforms in public, with the exception of going back and forth to work.
And security was stepped up within defence installations, requiring closer scrutiny of identification, the chief of defence staff said.
The extraordinary edict is tacit recognition that the shadowy threats faced so often in places like Afghanistan are present on home soil, requiring a sobering re-evaluation of measures the military takes to protect itself.
The order does not sit well, however, with some of the rank and file to whom the uniform is more than an article of clothing, but also a statement of pride and allegiance.
One of the soldiers who served with the slain Cpl. Nathan Cirillo on the memorial guard bristled at the suggestion the military was keeping a lower profile.
"I'm not afraid to wear my combats in public," said Lt. William McArthur of the Cameron Highlanders, a reserve regiment that often guards the Tomb of the Unknown Soldier. "It has come down that we will not be wearing our combats in public, but let's just say we respect the decision and we won't be wearing our combats in public."
The "combats" referred to by McArthur are the green diffused-pattern everyday dress that is ubiquitous on the streets around National Defence headquarters and the nation's capital.
The directive, a copy of which was obtained by The Canadian Press, also includes dress uniforms. It follows not only the Ottawa shooting, but Monday's tragedy in Quebec, where one soldier was killed and another was injured when they were struck by a car driven by an extremist who had been looking to join the Islamic State of Iraq and the Levant.
The edict "prohibits" military members from stopping at grocery stores, taking public transit and even having lunch or coffee in public places while in uniform.
Such activities are still permitted when soldiers are wearing civilian attire.
The directive was issued shortly before Defence Minister Rob Nicholson and Lawson laid a wreath near the spot where Cirillo was gunned down.
The defence chief cast the steps as sensible precautions given evolving international threats. But he was also sensitive to the message the directive could send to both the contain public and enemies alike.
"Let me be clear, we will not hide," Lawson said, who noted the departure of more aircraft and personnel for the international bombing campaign in Iraq. "We will not be deterred, nor intimidated from our duties."
Yet, fellow members of the honour guard at the granite war monument sombrely paid their respects Thursday dressed in civilian clothes. And Lawson conceded that the honour guard program of keeping vigil at the monument is suspended until a full security review, in conjunction with Ottawa Police, is carried out.
The whole area remains cordoned off as a crime scene under heavy police guard.
Well-wishers are being kept at a distance by steel barricades but have begun scattering bouquets of flowers in Cirillo's memory.
One such impromptu memorial — flowers, a solitary candle, and a hand-written letter — was placed at the corner of Sparks Street, just metres from where the shooting took place.
It thanked Cirillo for his service and lamented that his senseless death was unfair.
"Rest in peace, Nathan, wherever you are," said the note, which was tucked underneath a fan of a dozen yellow roses. "You have the whole nation standing with you forever and always."
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