The security detail was assigned following a threat assessment by the RCMP after a break-in at Trudeau's Ottawa home.
Last weekend, an intruder broke into Trudeau's home and left a threatening letter on top of several large kitchen knives while his wife and children were sleeping. Trudeau was in Winnipeg when the incident occurred.
A senior Liberal said there are now high-visibility RCMP patrols around the family's home.
An RCMP source said it was "unusual" to assign a protective detail to a political rally for an opposition leader.
The Liberals had planned for at least 600 people to turn out at a park overlooking the city's river, but they appeared to fall slightly short of that number.
Earlier Tuesday, Trudeau wouldn't say if the government should provide him with a security detail following the break-in but said he'd accept the expert advice given to him by the Mounties.
The risk assessment was done at the Liberal leader's request following the break-in overnight Saturday.
"I am looking forward to hearing recommendations the RCMP makes around my security, around my family's security," Trudeau told reporters gathered in Edmonton for the party's summer caucus. "I look forward to having conversations with them about what it will look like.
"I certainly am not going to second-guess the RCMP's expertise and look forward to the results of their investigation."
Under the current rules, only the prime minister and Governor General receive an automatic security detail. An exception is made during a federal election campaign, when the opposition party leaders are given security protection.