Video from the event shows Trudeau, and cabinet minister Ahmed Hussen, kneeling while surrounded by a crowd.
Trudeau clapped and nodded in response to many of the things said by speakers at the event, including one who said there is no middle ground on the issue.
“You are either a racist or an anti-racist,” the speaker said.
Trudeau gave a similar response when the people starting chanting: “Black lives matter.”
He did not speak and left as others began leaving Parliament Hill to start marching through the capital.
Trudeau declined to say on Friday morning whether he would be present at the rally.
The protest was one of many scheduled for Canadian cities in wake of the death of George Floyd, a Black man who died after a police officer held a knee to his neck during an arrest in Minneapolis, Minn., on the U.S. Memorial Day holiday.
In Toronto, Police Chief Mark Saunders also took a knee during a protest held earlier on Friday.
–With files from Global News