Image copyright Anadolu Agency via Getty Images Image caption Vigils were held across the country on Wednesday with more planned

Ukraine International Airlines flight PS752 was shot down "unintentionally" shortly after taking off from Iran's capital on Wednesday, Iran says.

All 176 passengers and crew were killed, including 57 Canadian citizens.

The plane was bound for the Ukrainian capital, Kyiv, and many on board had a connecting flight to Toronto, Canada's largest city and a transit hub.

Canadian Prime Minister Justin Trudeau has said that, all told, 138 people on that flight were en route to Canada.

Why were there so many Canadians on board?

Canada is home to a large Iranian diaspora, with some 210,000 citizens of Iranian descent, according to the latest federal census.

The country is also a popular destination for Iranian graduate and postdoctoral students to study and conduct research abroad, which is why many students were on the flight, returning to university following the winter break.

There is also no direct flight between Canada and Iran, and the Ukraine International Airlines flight from Tehran to Kiev and then to Toronto is popular because it is one of the most affordable options for the journey.

Media playback is unsupported on your device Media caption What do we know about the crash of Ukrainian Flight PS752?

Other victims of the crash included 82 Iranians and 11 Ukrainians, as well as nationals from Sweden, the UK, Afghanistan and Germany.

This is not the first air disaster that has touched Canada recently.

Last year, 18 Canadians were killed when an Ethiopian Airlines flight from Addis Ababa to Nairobi that crashed in March.

Who were the Canadian victims?

They were newlyweds, families, students, professionals and academics.

Image copyright Reuters Image caption Pedram Mousavi and Mojgan Daneshmand with their two daughters

Sixty-three of them were Canadian nationals, but many more called Canada their home, at least temporarily.

They lived in cities like Montreal, Toronto, Vancouver, Winnipeg, Ottawa and Edmonton, making this a truly national tragedy.

Many were in the 20s and 30s, but a number of young children were also on board.

At least two families died: Ardalan Ebnoddin Hamidi, Niloofar Razzaghi and their teenage son Kamyar, from Vancouver; and Pedram Mousavi and Mojgan Daneshmand, and their daughters Daria and Dorina Mousavi, from Edmonton.

There were numerous foreign students on board as well, like Ghanimat Azhdari - a PhD student at the University of Guelph, Ontario, who specialised in promoting the rights of indigenous groups. Why were so many Canadians on doomed Iran plane?

How are they being remembered?

Tributes were held across the country on Wednesday night, and many more are planned in the coming days.

In Toronto, a public vigil organised by the Iranian Canadian Congress was attended by over 100 people on Thursday. Flags will be lowered at city hall until Monday.

Another vigil was hosted on Parliament Hill in the Canadian capital of Ottawa, and attended by the prime minister. On Friday, he is meeting with family members of the victims of the plane crash.

Ukraine's embassy in Canada has invited the public to sign a book of condolences.

In Edmonton, Alberta, members of the Iranian-Canadian community estimate about 30 people who lived in the city were on flight PS752.

Media playback is unsupported on your device Media caption "She was full of dreams and now they're gone": Vigil held for Iran plane crash victims in Toronto

A memorial for Sunday is being organised by an Iranian heritage group, an Iranian student association, and the University of Alberta, an institution that lost a number of its faculty members and students in the crash.

In fact, the deaths have cast a pall over university campuses across the country. Many of the deceased are being remembered as accomplished teachers, students and mentors with bright futures who were studying and working at institutions of higher learning across the country.