Aaron Li-Hill was riding the streetcar on Friday evening, along with his girlfriend. He also had his bicycle with him.
Li-Hill said the couple were standing near the rear door of the streetcar, and that a man and a group of young women were sitting behind him.
Eventually, Li-Hill said, he heard the young women begin to scream.
"We got on near Kensington Market, and at just past Grace Street, there was, like a really, really loud scream, like a horrific shriek," Li-Hill told CBC News in an interview on Tuesday.
"And that's when I turned back and I saw one of the three girls like move away, to kind of get away. And I looked over and I saw the guy had a knife in his hand and he was pulling it out and sort of, like leaning forward."
Li-Hill said the three young women then ran "as fast as they could to the other side of the streetcar."
Li-Hill said he then turned his bicycle around and put it between himself and Yatim.
He remembers Yatim holding the knife away from his body and saying: "Nobody get off the f--king streetcar."
Li-Hill began backing up slowly, he said, and remembers seeing a number of passengers crowded against the door.
The streetcar came to a stop and the people on board flooded out.
"I'm like, slowly backing up and saying to him: 'Please, just let us get off the streetcar,'" Li-Hill said.
Li-Hill said that Yatim kept the knife upright and never pointed it directly at him.
When he backed down off the streetcar, Li-Hill said he asked the driver if he was OK, or if he wanted him to call 911.
"The last I saw was him standing there, with the streetcar driver sitting in the seat still."
A newly released video suggests that Toronto police officers arrived at the scene just moments after the passengers fled the streetcars shortly before midnight on Friday.
The 18-year-old Yatim was fatally shot soon after.
The shooting is currently being investigated by Ontario's police watchdog.
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