The man accused of ploughing a rental van into pedestrians on a crowded Toronto footpath, killing 10 people in Canada's deadliest mass killing in decades left a "cryptic message" on social media before his attack, police say.

Key points: Alek Minassian looks down as prosecutors announce charges

Alek Minassian looks down as prosecutors announce charges Justin Trudeau says Canadians should not live in fear

Justin Trudeau says Canadians should not live in fear Makeshift memorial grows near scene of attack

Suspect Alek Minassian, 25, was also charged with 13 counts of attempted murder for the incident that had the hallmarks of deadly vehicle assaults by Islamic State supporters, but Prime Minister Justin Trudeau said there was no reason to suspect any national security connection.

One possible clue to his motive emerged as Facebook confirmed Mr Minassian said in a post before the incident that referenced an "incel rebellion," referring to a shorthand used in some online message boards for "involuntary celibacy."

The post also voiced admiration for a man who killed six college students before taking his own life in California in 2014.

"The accused is alleged to have posted a cryptic message on Facebook minutes before" the attack, Graham Gibson, a Toronto police detective sergeant, told a news conference.

Suspect's link to incel: Mr Minassian's posting praised Elliot Rodger, a 22-year-old who killed six people in 2014 in California and who posted YouTube videos saying he was motivated by rage against women. Rodger had posted three videos on YouTube titled Spring break sucks when you're lonely, My reaction to seeing a couple at the beach ... envy, and Retribution. In the clips he said he was determined to punish women who had rejected him. Reddit, which hosts online chatrooms and bulletin boards, banned a group with 40,000 members called r/Incels in November. It said the group had violated its policy that prohibits calls for violence. Some members had lauded Rodger as a hero of the incel movement. Mr Minassian's Facebook posting read in part: "The Incel Rebellion has already begun". Reuters

The majority of the victims were women, ranging in age from their mid-20s to early 80s, he said.

He declined to answer a question about whether anger against women motivated the attack, saying, "that's going to be part of our investigation".

Facebook has since deleted Mr Minassian's account, a representative said.

"There is absolutely no place on our platform for people who commit such horrendous acts," she said.

Mr Minassian kept his shaved head down during a brief court appearance in Canada's largest city, speaking quietly with a defence lawyer and stated his name in a steady voice when asked to do so.

Mr Trudeau called on all Canadians to stand united with Toronto as flowers and scrawled messages in multiple languages piled up at a makeshift memorial in the city's north end, an ethnically diverse neighbourhood of towering office buildings, shops, restaurants and homes.

Alek Minassian has appeared in a Toronto court. ( LinkedIn )

"We cannot as Canadians choose to live in fear every single day as we go about our daily business," Mr Trudeau told reporters outside Parliament in Ottawa.

The Prime Minister said the incident had not changed the country's threat level or security preparations for a G7 summit in Quebec in June.

Sorry, this video has expired Prime Minister Justin Trudeau says Canadians should not live in fear

The Canadian flag was lowered to half-mast at Parliament and at Toronto City Hall.

Mr Minassian had briefly served in Canada's armed forces in late 2017 but asked to be voluntarily released after 16 days of training, defence ministry spokeswoman Jessica Lamirande said.

He had previously attended a high school program where one classmate remembered him as "absolutely harmless".

Accused Alek Minassian (second from left) makes his first court appearance. ( AP: Alexandra Newbould/The Canadian Press )

The suspect's two-story, red-brick home in a suburb north of Toronto was a crime scene on Tuesday, taped off and surrounded by police vehicles.

Officers went in and out of the house.

South Koreans among victims

Details about the dead began to emerge on Tuesday, with a South Korean foreign ministry representative saying two of that country's citizens were killed and one injured in the attack.

The representative spoke on condition of anonymity because he was not authorised to speak to the news media.

The attack happened during a busy time on Toronto's Yonge Street. ( AP: Nathan Denette/ Canadian Press )

The Canadian Broadcasting Corporation identified one of the victims as Anne Marie D'Amico, an employee of asset manager Invesco Canada.

In a statement, Invesco confirmed one of its employees had been killed but did not name her.

The attack shook the usually peaceful streets of Toronto, a multicultural city with a population of 2.8 million. The city recorded 61 murders last year.

The drama started at lunchtime on a warm spring day, when the driver drove his vehicle into the crowds.

The street was soon covered in blood, empty shoes and bodies

Reuters