There was a heavy police presence at a Liberal rally in southern Ontario today.

Citing unnamed senior Liberal sources, CBC News reported that this was in response to a security threat against Justin Trudeau.

The Liberal leader was even wearing a bulletproof vest when he spoke to a crowd of about 2,000 people in Mississauga. 

The event was delayed by an hour-and-a-half and Trudeau didn't hang around afterward as long as he normally does to pose for selfies.

Trudeau's wife Sophie was scheduled to introduce him but she did not go on-stage.

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Earlier this year, the then clerk of the privy council, Michael Wernick, told the Commons justice committee that he was alarmed about the possibility of someone being shot during the 2019 election campaign.

"I worry about the rising tide of incitements to violence when people use terms like treason and traitor in open discourse," Wernick said at the time.

There have been very few political assassinations in Canadian history, though there was an attempt against Pauline Marois on the night she was elected premier of Quebec in 2012. The gunman killed lighting technician Denis Blanchette and seriously wounded stagehand David Courage.

Former Quebec Labour minister Pierre Laporte was murdered by the Front de Libération du Québec in 1970 during the October Crisis. And one of the fathers of Confederation, George Brown, died from a gunshot wound in 1880.

He was publisher of the Globe and Mail and a senator at the time of the shooting.