The date and time of the rescheduled game will be announced later, the NHL said in a statement.
A Canadian Forces soldier at the National War Memorial was killed Wednesday morning by a masked gunman, who then moved up the street to launch an attack on nearby Parliament Hill, in which at least two people were injured. The assailant was shot dead inside parliament's Centre Block.
"We obviously respect the league's decision to cancel the game," Maple Leafs general manager Dave Nonis told reporters in Ottawa. "Today's events far outweigh a hockey game. We look forward to coming back and playing the game when they see fit to reschedule it. Our thoughts go out to all the people affected, the police force that you can hear working so hard all day. A hockey game's definitely secondary today."
Senators owner Eugene Melnyk released a statement saying he knows fans stand with the organization to support the league's decision.
"We are shocked and deeply saddened by today's tragic events on Parliament Hill and in downtown Ottawa," Melnyk said in the statement. "Our collective thoughts and prayers are with the families of the victims as well as with all members of Parliament and staff who have had to manage through today's difficult circumstances.
It's the second time in less than two years that the NHL has postponed a game due to a violent incident. In April 2013, the league NHL cancelled a game between the Bruins and Senators after the Boston Marathon bombing. Those teams played the game the day after the regular season ended.
"The National Hockey League wishes to express its sympathy and prayers to all affected by the tragic events in Ottawa," the league said in its statement.
The Leafs, who played the Islanders last night in New York, did not have a morning skate at Canadian Tire Centre.
"The Toronto Maple Leaf Hockey Club extends its deepest condolences to the family and friends of all affected by the tragic events in downtown Ottawa today," the team said in a statement. "We have been witness today to some terrible events but are thankful to those brave men and women for their tireless service in protecting everyone's safety."
A spokesman said the team was safe at its hotel near Parliament Hill at the time of the shootings.
"Surreal scene outside of our hotel right now," Leafs forward Joffrey Lupul tweeted. "Lot of very brave police officers we should all be very proud of."
Lupul is a supporter of Canada's military with his "Lupe's Troops" program, which donates two tickets to every home game to current members of the Canadian Forces, who are also honoured at the game. He later posted a picture of the Parliament Buildings on Instagram with the caption "Strong."
Nonis said before leaving the team's hotel that the league was in communication early on to inform the organization that the game could be postponed. The message was to be prepared to play, and from his vantage point players handled the situation well.
"We're very fortunate to be in a safe environment here," Nonis said. "You think more about what's going on outside, and you hear a lot obviously from the sirens and what you follow on television that it was a serious and significant event."
The Senators held their regular pre-game skate at Canadian Tire Centre, which is located about 30 minutes outside of the downtown core.
"Makes me sad what's going on in Ottawa right now," Senators captain Erik Karlsson tweeted. "Thoughts and prays to everyone involved. Everybody stay safe."
Senators veteran Chris Phillips added: "Heartfelt thanks to all first responders for putting their safety on the line for ours. #OttawaStrong."
The Senators' next home game is scheduled for Saturday against the New Jersey Devils.
The Leafs left Ottawa on Wednesday afternoon to fly back to Toronto. Their next game is Saturday against the Boston Bruins.
With neither team scheduled to play Thursday and the arena available, it was an option for the rescheduled game. But Nonis agreed with the NHL opting to decide on another date in the near future.
"I think that the situation such as it is that it was better that they distanced themselves more than 24 hours," Nonis said. "At the time they cancelled the game, who knows how long it was going to go on for. We'll find a time to play this game."
The Pittsburgh Penguins' game against the Philadelphia Flyers replaced Leafs-Senators on national television. Even though two American teams were facing off, "O Canada" played before the "Star-Spangled Banner" at Consol Energy Center. A red and white Maple Leaf was projected on the ice during the anthem, with the Canadian flag displayed on screens around the arena.
In Edmontton, the Oilers had a moment of silence before their game against the Washington Capitals, recognizing both the shootings in Ottawa and the attack in Quebec earlier this week. The Anaheim Ducks also had a moment of silence before their game against the Buffalo Sabres.
The Toronto Raptors also announced they would have extra security in place for Wednesday night's pre-season game against Maccabi Haifa. The Raptors held a moment of silence prior to tip off.
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