OTTAWA - A Canadian honour guard who witnesses said was shot twice "point blank" by a gunman at the National War Memorial has died, while police continue combing a locked-down national capital for other potential threats.

The unnamed reservist from Hamilton, Ont., was killed by a masked, rifle-wielding gunman, who then moved up the street to launch an attack on nearby Parliament Hill, in which at least two people were injured.

A security guard is believed to be among those who were wounded in Parliament's Centre Block, where an assailant was shot dead by the sergeant-at-arms of the House of Commons. No details about the second injured person were immediately available.

However, security staff on the Hill have told some MPs' offices they believe there may be as many as three shooters. Heavily armed tactical officers could be seen searching rooftops in the immediate parliamentary precinct.

Police would only say they are investigating "several shooting incidents in downtown Ottawa."

The incident comes just two days after two Canadian soldiers were run over — one of them fatally — in Quebec by a man with jihadist sympathies.

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

Tony Zobl, 35, witnessed the soldier being gunned down at the War Memorial from his fourth-floor office window directly above the monument.

"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," said the witness.

"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.

The Langevin Block, home of the Prime Minister's Office across the street from Parliament Hill, has been evacuated. All three party leaders are confirmed to be in safe locations, party officials say.

Police herded bystanders off the street into a major office building and warned people to stay away from the windows. They have told people in the precinct that if they cannot lock their doors, they should barricade them.

The U.S. embassy, adjacent to Parliament Hill, has also been locked down, and U.S. President Barack Obama has been briefed on the ongoing incident.

Two sources told The Canadian Press that Kevin Vickers, the sergeant-at-arms for the House of Commons, shot an assailant inside the Hall of Honour, the main entrance to the Centre Block beneath the Peace Tower.

Prime Minister Stephen Harper was rushed away from the building to an undisclosed location.

"Our thoughts and prayers are with the families of those who were attacked," Harper's communications director Jason MacDonald said in a statement.

"The prime minister is safe and not on Parliament Hill and being briefed by security officials."

Ottawa police confirmed they had a call at 9:52 a.m. with a report of shots fired.

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

Walsh says 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 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 dark car at gunpoint, he said, and drove it up towards the Peace Tower.

A second vehicle, with no rear licence plate, was parked directly in front of the Langevin Block in a no-parking zone. Sources say the vehicle remains a source of concern to authorities.

Bomb-detecting robots have been deployed in the area.

Cabinet minister Tony Clement tweeted that at least 30 shots were heard inside the building, where Conservative and Liberal MPs were holding their weekly caucus meetings.

Karl Belanger, a senior aide to NDP Leader Tom Mulcair, says security told them to remain locked in the Opposition leader's office as there may be two or three gunmen.

Witnesses said shots were fired in several different corridors inside the Centre Block, the central parliamentary building that includes the Peace Tower and the House of Commons. Some people fled the building by scrambling down scaffolding erected for renovations.

Chuck Bronley of Ottawa said he was near the memorial when he heard four shots and saw a man running away carrying what he said was a rifle.

Jan Lugtenborg, a tourist from the Netherlands, described the shooter as having very long, black hair.

"We heard four shots," said Lugtenborg. "You don't expect that when you're on holiday in Canada."

Police were yelling at bystanders, shouting that there was still a shooter on the loose.

"DND member is down," one officer was overheard saying.

Bronley and Lugtenborg said the victim appeared to be one of the ceremonial guards at the War Memorial.

Two people vigorously pumped the victim's chest on the stairs of the War Memorial, until the victim was finally placed on a stretcher and ushered into an ambulance shortly after 10 a.m.

— With files from Jennifer Ditchburn, Steve Rennie, Jim Bronskill, Murray Brewster, Stephanie Levitz, Joan Bryden, Andy Blatchford and Lina Dib