Justin Trudeau has said the Canadian government "will not rest until we get answers" into the downing of a Ukrainian passenger plane by an Iranian missile.
All 176 people on board the flight from Tehran on Wednesday were killed, including 138 who were headed for Canada.
Iran has admitted the plane was mistaken for a hostile target amid soaring tensions with the United States.
Speaking at a memorial in Edmonton, Alberta, Mr Trudeau said the government "will not rest until there is justice and accountability".
Iran denied for three days that it was the cause of the crash, initially blaming engine failure.
Mr Trudeau said it has been gut-wrenching to listen to stories from relatives of 57 Canadians who died in the downing of a Ukrainian jetliner in Iran.
He said he has learned many of the victims came to Canada in search of new opportunities for their families, but those families are now consumed by grief and outrage.
Mr Trudeau called it a Canadian tragedy.
“This tragedy should never have occurred,” he said.
“We will not rest until there are answers.
"We will not rest until there is justice and accountability.”
Other memorials were held across the country.
At the Vancouver Art Gallery, national defence minister Harjit Sajjan also called the crash a national tragedy.
At the University of Toronto, many cried throughout the ceremony as speakers listed victims, including a one-year-old child.
Fati Mortazavi, whose best friend died in the crash, said having a community come together helped her cope with the tragedy.
Foreign Affairs Minister Francois-Philippe Champagne said Canada's Standing Rapid Deployment Team arrived in Iran on Saturday to "provide consular assistance to the families of the victims" and to "assist in the investigation."
Meanwhile, online videos have emerged purporting to show Iranian police and security forces firing live ammunition and tear gas to disperse demonstrators protesting against the Islamic Republic’s initial denial that it shot down the plane.
International rights groups have called on Iran to allow people to protest peacefully as allowed by the country’s constitution.