Speaking at a memorial with a capacity crowd of 2,300 in Edmonton, Alberta, Trudeau 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.
The plane was shot down by an Iranian missile moments after taking off from Tehran on Wednesday. All 176 on board 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.
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 Sunday across the country.
At the Vancouver Art Gallery, National Defense 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. People broke out in raucous applause several times when various speakers and politicians said Iran would be held accountable.
Fati Mortazavi, whose best friend died in the crash, said having a community come together helped her cope with the tragedy.
"It's so comforting for us," said Mortazavi. "As long as we know that someone cares for these people, that's so important for us."
Deputy Prime Minister Chrystia Freeland attended the vigil in Toronto and said the loss goes well beyond the university.
"This is Toronto's loss, this is Ontario's loss and this is Canada's loss," an emotional Freeland told the vigil. "Nothing will ever replace these brilliant lives that have been cut short. We will always ... bear these scars."
Three members of Canada's standing rapid deployment team arrived Saturday in Iran to establish a base of operations for the Canadian government in the wake of the crash.
Foreign Affairs Minister Francois-Philippe Champagne said Sunday that Iran has approved visas for six more members of the team, who are currently in Ankara, Turkey, as well as for two experts from the Transportation Safety Board.
The eight are to travel to Iran on Monday, Champagne said in a series of tweets.
A spokesman for Champagne said the officials "will be there to provide consular assistance to the families of the victims, including supporting repatriation of remains, to help identify victims and to assist in the investigation."
Canada's Transportation Board said Sunday it also plans to deploy a second team of investigators who specialize in aircraft recorder download and analysis.