A man driving a pickup truck rammed into and killed four members of a Muslim family in the south of Canada's Ontario province, in what police said on Monday was a “premeditated” attack.

A 20-year-old suspect wearing a vest “like body armour” fled the scene after the attack on Sunday evening, and was arrested at a mall seven kilometres from the intersection in London, Ontario where it happened, said Detective Superintendent Paul Waight.

“There is evidence that this was a planned, premeditated act, motivated by hate. It is believed that these victims were targeted because they were Muslim,” he told a news conference.

The names of the victims were not released, but they include a 74-year-old woman, a 46-year-old man, a 44-year-old woman and a 15-year-old girl — together representing three generations of the same family, according to London Mayor Ed Holder.

A nine-year-old boy was also hospitalised following the attack and is recovering.

“Let me be clear, this was an act of mass murder perpetrated against Muslims, against Londoners, rooted in unspeakable hatred,” said Holder.

Holder said flags would be lowered for three days in London, which he said has 30,000 to 40,000 Muslims among its more than 400,000 residents.

Identified as Nathaniel Veltman, the suspect has been charged with four counts of first degree murder and one count of attempted murder.

Police said Veltman, a resident of London, did not know the victims.

Waight said local authorities are also liaising with federal police and the attorney general about adding “possible terrorism charges”.

He offered few details of the investigation, but noted that the suspect's social media postings were reviewed by police.

Waight said police did not know at this point if the suspect was a member of any specific hate group and declined to detail evidence pointing to a possible hate crime, but said the attack was planned.

Canadian Prime Minister Justin Trudeau tweeted that he was “horrified” by the attack.

“To the loved ones of those who were terrorised by yesterday's act of hatred, we are here for you,” he said, singling out the nine-year-old in hospital.

“To the Muslim community in London and to Muslims across the country, know that we stand with you. Islamophobia has no place in any of our communities. This hate is insidious and despicable — and it must stop,” he added.

'Condemnable act of terrorism'

Calling it a "condemnable act of terrorism", Prime Minister Imran Khan said the incident revealed growing Islamophobia in Western countries. "Islamophobia needs to be countered holistically by the international community," he said.

Foreign Minister Shah Mahmood Qureshi said three generations of Pakistani-Canadians were killed in a "brutal act of mass murder for their Muslim faith".

"This is an act of terror rooted in unspeakable hatred and Islamophobia. Extend our deepest sympathy to the family members and pray for recovery of the only survivor; a little boy," he said.

In a statement issued on Tuesday, the Foreign Office (FO) condemned the Islamophobic incident that resulted in the deaths of the four victims.

"The High Commission for Pakistan in Ottawa and the Consulate General in Toronto are in close contact with the relevant Canadian authorities to ascertain the facts of the case and to ensure that the perpetrators of this heinous act are brought to justice," the FO statement said.

The consul general in Toronto also visited the family of the deceased to offer condolences and all possible assistance, it added.

"This tragic incident is yet another manifestation of the systematic rise in Islamophobia. The Canadian prime minister has stated that Islamophobia has no place in any of their communities. Events like these further reinforce the imperative for the international community to work together for interfaith harmony and peaceful coexistence," the statement said.

'Out for a walk'

At about 8:40pm on Sunday, according to police, the five family members were walking together along a sidewalk when a black pickup truck “mounted the curb and struck” them as they waited to cross the intersection.

One woman who witnessed the aftermath of the deadly crash said she couldn't stop thinking about the victims. Paige Martin said she was stopped at a red light around 8:30pm when a large pick-up roared past her. She said her car shook from the force.

“I was shaken up, thinking it was an erratic driver," Martin said.

Minutes later, she said, she came upon a gruesome, chaotic scene at an intersection near her home, with first responders running to help, a police officer performing chest compressions on one person and three other people lying on the ground. A few dozen people stood on the sidewalk and several drivers got out of their cars to help.

"I can't get the sound of the screams out of my head," Martin said.

From her apartment, Martin said she could see the scene and watched an official drape a sheet over one body about midnight. "My heart is just so broken for them," she said.

Zahid Khan, a family friend, said the three generations among the dead were a grandmother, father, mother and their teenage daughter. The family had immigrated from Pakistan 14 years ago and were dedicated, decent and generous members of the London Muslim Mosque, he said.

“They were just out for a walk that they would go out for every day,” Khan said through tears near the site of the crash.

A fundraising webpage said the father was a physiotherapist and cricket enthusiast and his wife was working on a PhD in civil engineering at Western University in London. Their daughter was finishing ninth grade, and the grandmother was a pillar of the family, the page said.

Qazi Khalil said he saw the family on Thursday when they were out for their nightly walk. The families lived close to each other and would get together on holidays, he said.

"This has totally destroyed me from the inside," Khalil said. "I can't really come to the terms they are no longer here."

The attack, which brought back painful memories of a Quebec City mosque mass shooting in January 2017 and a driving rampage in Toronto that killed 10 people in April 2018, drew swift condemnation.

The National Council of Canadian Muslims said in a statement it was “beyond horrified and demands justice” for the family who were just “out for a walk” on a warm spring evening.

“This is a terrorist attack on Canadian soil and must be treated as such,” its president, Mustafa Farooq, told Radio Canada.

The Muslim Association of Canada also called on authorities to “prosecute this horrific attack as an act of hate and terrorism”.

“Hate and Islamophobia have NO place in Ontario,” tweeted Ontario Premier Doug Ford. “These heinous acts of violence must stop.”

Four years ago, a 27-year-old white supremacist burst into a Quebec City mosque and unleashed a hail of bullets on worshippers who were chatting after evening prayers, killing six men and seriously wounding five others.

At the time, prior to New Zealand mosques shootings in March 2019, it was the worst ever attack on Muslims in the West.

The shooter, Alexandre Bissonette, was sentenced to 40 years in prison, but that was lowered on appeal, and the Supreme Court is now reviewing his punishment.

Meanwhile, a 28-year-old man who ploughed a rented van into pedestrians at high speed three years ago in Toronto was found guilty in March of murdering 10 people and trying to kill 16 others.

Before that attack, Alek Minassian posted on Facebook a reference to an online community of “involuntary celibates” whose sexual frustrations led them to embrace a misogynist ideology.

He is to be sentenced in January 2022.

Additional input from Naveed Siddiqui