Nine people are dead and 16 injured after a van hit pedestrians in the Canadian city of Toronto.
Witnesses say the vehicle swerved across four lanes to hit passers-by "deliberately".
Five of those hurt are critical, police have confirmed.
Officers say they have arrested the driver of the van but have not given any details on age, gender or possible motive.
Footage from the scene appears to show a man being arrested.
Police were called just before 1.30pm local time (7.30pm Irish time) to the corner of Yonge Street and Finch Avenue, a busy area north of downtown Toronto, where the van mounted the pavement and hit pedestrians.
Mon Apr23 1:45pm police responded to van striking multiple pedestrians Yonge St & Finch St Toronto~At 4:30pm press conference Deputy Chief Peter Yuen confirmed 9 people are dead & 16 injured~Van driver arrested~POLICE ASK PUBLIC TO REFRAIN FROM CALLING 911 UNLESS EMERGENCY ^sm pic.twitter.com/ZyAJS8rQDv
— Toronto Police (@TorontoPolice) April 23, 2018
Witnesses have said the van was "fairly deliberately" hitting pedestrians and that the collision looked like it was "on purpose".
Henry Miller, who works from home for a bank in Toronto and lives in an apartment overlooking the street, said: "I heard a large amount of honking... and I stepped out on to the balcony to see what was causing the trouble and at this point I saw a lady lying on the floor and the van driving off with people running around trying to help her."
The van "continued to swerve in and out of traffic all the way down", said Mr Miller, who witnessed about 30 seconds of the incident.
He said the van swerved across four lanes at speed.
He described a white van, "speeding probably 60-70mph down this major road, swerving to what I would say fairly deliberately hit pedestrians and swerving in and out of traffic before eventually careering off into one of the other side streets and out of view".
Mr Miller says there are "lots of police and fire engines and ambulances everywhere" following the crash.
"I've not left my apartment yet, out of concern for my own safety," he said.
Some streets, buildings and the subway in the area have been shut down.
A second witness, Diego DeMatos, was driving on Yonge Street in the opposite direction to the van and witnessed the vehicle collide with pedestrians.
"We saw the last few people get hit when we were driving north on Yonge Street, which is one of the busiest streets in Toronto," he said. "I'm still a little bit shaken so sorry if I don't speak properly.
"We actually saw when the van hit the last few people that were on the sidewalk. We thought it was just a hit and run. He kept driving… all I could see was dead bodies on the floor.
"As I was driving up north on Yonge I could see more and more people lying on the ground. Then paramedics started to arrive and the police started to arrive so I wasn't really sure what was happening.
"I stopped to help one of the victims on the side of the road along with a few other people. It was a hard image to see.
"It was on purpose. I don't know if he was drunk, I don't think so, or if he maybe had a medical condition that happened. He was on the sidewalk and then on the road and then on the sidewalk and [the people] were all a few feet from each other. Maybe it continued for about six blocks or so."
At least seven people were taken to Sunnybrook Health Services Centre's trauma centre, the hospital said on Twitter.
A statement from the hospital says: “We currently have five people in critical condition, two in serious condition, one in fair condition.”https://t.co/vlpg0Rod7v
— Jack Quann (@jqbilbao) April 23, 2018
Ryder, a rental and maintenance firm which operates one of North America's largest fleet of trucks, according to its website, has confirmed one of its vehicles was involved in the incident and is cooperating with authorities.
The collision happened about 30 kms from downtown, where the G7 foreign ministers were meeting on Monday.
There was no noticeable change in security around the Intercontinental Hotel, where the ministers gathered.
Canadian Prime Minister Justin Trudeau said officials were investigating.
"We're still gathering information and as soon as we can, we'll share more information with Canadians," Mr Trudeau told reporters about an hour after news of the incident broke.
He released a statement on Twitter, saying: "Our thoughts are with all those affected by the terrible incident at Yonge and Finch in Toronto. Thank you to the first responders working at the scene - we're monitoring the situation closely."
Our thoughts are with all those affected by the terrible incident at Yonge and Finch in Toronto. Thank you to the first responders working at the scene – we're monitoring the situation closely.
— Justin Trudeau (@JustinTrudeau) April 23, 2018
Toronto Police Services Deputy Chief Peter Yuen confirmed that nine people had died and 16 were injured.
He said officers were interviewing witnesses and examining surveillance video as part of the "complex" investigation.
"I can assure the public all our available resources have been brought in to investigate this tragic situation," he said.