Just as a news conference was ending at a meeting of Canada's justice ministers, MacKay interrupted saying he heard that Trudeau had discussed legalizing marijuana in front of school children in Brandon, Man.
"Now I could see Justin Trudeau coming before an assembly like ours, or going before a police convention, or going before a group of even college students. But to discuss this subject matter in front of children, some of them preteens, about his proposal to legalize marijuana, I find just appalling," MacKay said.
While Trudeau's comments were made at the Sioux Valley First Nations elementary school, Jillian Austin, a reporter for the Brandon Sun newspaper, said the Liberal leader was speaking to a group of teenage high-school students in the gym.
Trudeau's statement on marijuana was a response to a question from a student, she said.
Austin said Trudeau started by saying that marijuana was dangerous for young people, because their minds are still developing, but that he believes regulating pot will make it safer for children.
The high school students had come to the elementary school for the event, said the school.
MacKay said the comments reflect poorly on Trudeau.
"We have no intention of decriminalizing marijuana. And he can shout this from the hilltops as much as he likes, but going before school children, in my view, crosses the line of appropriate behaviour for a federal leader."
Trudeau issued a statement late Thursday calling on MacKay to retract his comments.
"The students in the room applauded a politician with a message to stay off drugs, and that the current system is not doing enough to keep it out of the hands of kids," the statement said.
"That the Conservatives would put out a statement condemning the courage showed by those students is shameful."
Before MacKay criticized Trudeau, justice ministers from all provinces and territories gathered for a two-day meeting in the Yukon capital.
MacKay said they discussed a wide range of issues around justice and public safety, such as impaired driving, cyber crime, those with fetal alcohol syndrome in the justice system and funding formulas.
The minister noted the federal government is considering expanding consecutive sentencing terms to crimes other than murder.
"Certainly sexual offences, aggravated sexual assault, certain violence criminal offences that might involve weapons or aggravated circumstances and child-sex offences are other Criminal Code provisions we're looking at to potentially expand the provisions where consecutive versus concurrent sentencing would apply."
The Multiple Murders Act now allows judges to impose consecutive terms of parole ineligibility for those who commit more than one murder.
Ontario's Attorney General John Gerretsen said everyone at the meeting has concerns about resources, particularly in supporting a legal aid system.
"I think access to justice is probably the most predominant issue with most of the ministers," he said.
"When you realize the fact that many people who appear in our criminal courts, our family courts are unrepresented, access to justice and what legal aid can do to improve the necessary representation is always an issue."
Host, Yukon Justice Minister Mike Nixon, said the ministers also have concerns about offenders in the correction system who have fetal alcohol spectrum disorder.
He said a prevalence study is being conducted in Yukon right now, but he couldn't say if that study would result in those with FASD being labelled as having a disorder.
Late Thursday, the Conservative party, on behalf of Peter MacKay, put out a statement to its members decrying Trudeau's "reckless plan to legalize marijuana."
The statement asked supporters for $25, or whatever they could afford, to help Conservative candidates have a strong showing in byelections being held Nov. 25 in four federal ridings, in Ontario, Quebec and Manitoba.
Editor's note: This story contains a clarification. An earlier story did not specify that a reporter at the event said Justin Trudeau's marijuana remarks came when speaking to high school students.