So as I'm putting it together now with a couple hours elapsed- this is all what I can find open source and from photos/videos. I cannot confirm any of what I've got here, but this is what looks pretty solid right now, again, mostly drawn from photos, videos, and witness accounts to media.
It appears he was southbound on Yonge in a white rental van. He hopped the curb and started striking people, it appears that he left several people dead in his wake, this based on photos of what appear to be multiple orange tarps over victims. He continued down the sidewalk for about a kilometer, turned right onto Poyntz and got out of his car.
Initial police response seemed to be routine for a pedestrian hit and run- there's video of a cruiser rolling up to a scene, with one on the ground and a bunch of bystanders pointing him south down Yonge. He then takes off down Yonge street. Very quickly it sounds like word got out about multiple scenes.
There is video from where the van rounded the corner onto Poyntz. It appears to have stopped voluntarily and the driver exited. I saw no barriers, obstacles, or serious damage that would stop the van. There's further video of the first officer approaching the driver at gunpoint. The driver rapidly draws what I have read was a cellphone on the officer several times while yelling at the officer. I would bet a pay check he was trying to get shot based on the video of the repeated quick draw with the object. The officer did not fire, and it appears that the male was brought safely into custody.
Mass casualty response. Sunnybrook hospital has brought in at least eight victims, and the hospital announced one dead, five in critical, one in serious, one in fair. Now I believe but am not certain that this is in addition to likely more than one victim who were deceased and remained on scene under tarps.
It looks like there were pedestrian strikes in multiple places. It looks like he stayed on the sidewalk.
Toronto Police have lead, and Ontario Provincial Police and RCMP are supporting.
I've seen/heard more than that, but nothing I would characterize yet as stronger than rumor.
EDIT TO ADD: Toronto Police just confirmed nine dead.