A gunman turned the nation's capital into an armed camp Wednesday after he fatally shot an honour guard at "point blank'' range at the National War Memorial before setting his sights on Parliament Hill.

The extraordinary scene ended with the assailant shot dead in the polished marble halls of Parliament's Centre Block, apparently by the sergeant-at-arms of the House of Commons, while SWAT teams combed the busy parliamentary precinct in an ongoing but seemingly fruitless search for accomplices.

Slain reservist Nathan Cirillo of Hamilton, Ont., is the second member of the Armed Forces this week to die in an apparently random, murderous attack, just as Canadian war planes are being deployed to Iraq.

Two Canadian soldiers were run over — one of them fatally — in Quebec on Monday by a man with jihadist sympathies.

The slain shooter was identified as Michael Joseph Paul Zehaf Bibeau, born in 1982 and known to police in Montreal and Vancouver.

"It's way too early to be able to determine a motive,'' Gilles Michaud, the assistant commissioner of the RCMP, said at a news conference in Ottawa while police were still clearing downtown buildings.

Prime Minister Stephen Harper, who was in the Centre Block addressing a Conservative caucus meeting when the attack began just outside the door, was expected to make a statement to the country later Wednesday.

Tony Zobl, 35, witnessed Sgt. Cirillo being gunned down at the War Memorial from his fourth-floor office window directly above the monument, just before 10 a.m. Wednesday.

"I looked out the window and saw a shooter — a man dressed all in black with a kerchief over his nose and mouth and something over his head as well — holding a rifle and shooting an honour guard in front of the Cenotaph, point blank, twice,'' Zobl told The Canadian Press.

"It looked like the honour guard was trying to reach for the barrel of the gun,'' he continued. ``The honour guard dropped to the ground and the shooter kind of raised his arms in triumph holding the rifle.''

Zobl said the shooter then ran up the street toward Parliament Hill.

It was only earlier this year that the government extended the season for live guards at the monument through the November Remembrance Day ceremonies. In the past, the honour guard has stood down after Labour Day.

The scene on Parliament Hill was frantic.

Wednesday morning is the busiest day of the week on the Hill, with MPs and senators of all major parties gathering in the Centre Block for meetings.

Given the time and day, it was remarkable only four persons were admitted to hospital. One was Cirillo, and the other three were all released Wednesday evening. Only one had a minor gunshot wound.

Witnesses and video suggest a hail of gunfire erupted in the marbled Hall of Honour that bisects the building directly beneath the Peace Tower.

Kevin Vickers, better known to political wonks as the ceremonially garbed sergeant-at-arms carrying the mace to open the Commons each day, was reported by multiple sources as the person who shot the gunman.

Construction worker Scott Walsh said he was in a manhole near the East Block building, between the memorial and the Centre Block, when he heard two gunshots echo down the street.

In the ensuing panic, people around him started screaming, including a woman pushing a child in a stroller.

He said she started to run, so he went to help her.

That's when he saw a man with long black hair, his face covered by a white scarf with decals on it and wearing a black jacket.

"He had a double-barrelled shotgun, he was about five feet from me, and he ran right beside us, ran past the woman with the stroller and child,'' he said.

The gunman then hijacked a car at gunpoint, he said, and drove it up towards the Peace Tower.

Greta Levy, a press secretary for the NDP, said she had just left the building when she saw the gunman walk up the paved ramp under the tower.

"None of us reacted at first but then we heard a security guard yelling, 'There's a gun, get down, get down, there's a gun,''' Levy said. "And I looked up, as did the woman I was with, and saw a man that I would describe as young, 20s-30s, coming up the ramp as though to go in the main doors of Centre Block under the Peace Tower.''

The incident paralyzed the entire downtown core for hours, from the adjacent U.S. embassy to the University of Ottawa several city blocks to the south.

Sources say police at one point worried there could be up to five assailants, and they combed rooftops of nearby buildings in addition to the full lockdown.

The incident shattered the sleepy tenor of a capital that has been remarkably immune from attack through more than a decade of tumultuous overseas wars.

"I feared this day would come, and my prayers are with the fallen soldier,'' senior cabinet minister Tony Clement posted on Twitter while locked in the caucus room. "Hug your fam.''

"It's a stomach-turning, shocking day for all Canadians,'' Marc Garneau, a former Forces member and now Liberal foreign affairs critic, said in an interview.

The Hill is known as an open public space that's used for everything from mass yoga classes to hazy pot-legalization demonstrations.

"I think the intention was to try to make Parliament not look like Fort Knox,'' said Garneau. "But we've crossed a river today.''

Green party Leader Elizabeth May urged restraint.

"Today is not a day that 'changes everything,''' May said in a statement. "It is a day of tragedy. We must ensure we keep our responses proportionate to whatever threat remains.''

Military bases have been put on alert and soldiers have been cautioned about wearing uniforms in public.

In Toronto, the country's largest city, extra police where put on the streets and public buildings were on alert.

Jan Lugtenborg, a tourist from the Netherlands, was at the War Memorial and able to describe the shooter in detail.

"We heard four shots,'' said Lugtenborg.

"You don't expect that when you're on holiday in Canada.''