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The gunman entered the building as Members of Parliament gathered on the Hill for their weekly caucus meetings.
"Shots fired at War Memorial at 9:52am today; one person injured," Ottawa Police tweeted.
All buildings and federal government offices within the area are in lockdown, including those at the University of Ottawa.
"All Ottawa Police stations are closed to public due to shooting at War Memorial. No public services available until further notice," Ottawa Police tweeted 45 minutes later.
Government officials assured the public that Prime Minister Stephen Harper was safe and has been removed from the building. The Prime Minister's Office has declined to give further information for security reasons.
The incident occurred hours after Canada had raised its terrorist threat level after a fatal hit-and-run attack on two soldiers Monday.