It happened at about 2 a.m. on Bennett Street near Nelson Street.
Kyle MacLeod was sleeping on the 16th floor of a highrise building on Bennett Street, when he was startled by the noise.
"We heard some commotion, some shouting. We heard someone screaming that they had a knife. We looked through the window and saw a group of guys," said MacLeod.
"I thought it was a fight between two groups, but then as I watched, it looked like one group really putting a beating on one other person."
MacLeod said there were 10 to 12 people in the group, but only half of them appeared to be directly involved in the alleged attack.
Jin Pyeon was on the sixth floor of the same highrise watching television when he heard the disturbance.
"We saw about six or seven young men beating up one person in a white shirt, I think," alleged Pyeon.
"The young person tried to get away from them, starting off in front of The Bay store, and he was running away, but they caught him and bring him down to the ground, and they kicked him and hit him."
Pyeon said some of the alleged attackers hopped in a waiting car and tried to make a getaway, and several men ran away on foot.
Both MacLeod and Pyeon said they were alarmed by the incident.
"It was pretty shocking, actually. I mean, we're used to noises a bit around here at Metrotown, but we don't too often hear that," said MacLeod.
Burnaby RCMP said the victim's injuries were reported to be non-life-threatening.
Police wouldn't say if they had made any arrests, though a black sedan stopped nearby on Nelson Street was referred to as a vehicle of interest.