A gunman shot and killed a soldier at a war memorial in Ottawa, then ran into the parliament building where he exchanged gunfire with police, the BBC reported.
One gunman is dead, but parts of Canada's capital remain on lockdown as police hunt for more suspects.
At a press conference, police said the situation was "ongoing" and "fluid".
It came hours after Canada raised its terror threat level, after another soldier was killed on Monday in a hit-and-run attack by a Muslim convert.
The country earlier this month announced plans to join the US-led campaign of air strikes against Islamic State (IS) militants in Iraq.
But there is no confirmation any of this week's attacks are linked to IS or the new military campaign.
The latest incident began when soldiers guarding the memorial came under fire on Wednesday morning.