Toronto police say an accidental bumping of shoulders at Yonge-Dundas Square set off an assault that resulted in one man being charged with attempted murder and another taken to hospital with a life-threatening stab wound.

The incident happened at around 12:30 a.m. Police say the victim, a 33-year-old man, was walking and texting on his phone when he accidentally bumped into another man, who attacked him and was accompanied by another man.

Police say the man was able to defend himself and that his attackers "took exception to the defence."

"He inadvertently bumped shoulders with one of the two male suspects," said Det. Jason Shankaran. "This perceived slight was taken very seriously by these two males, and they attacked him. Unknown to the two suspects was that the victim was very able to protect himself."

One of the two suspects had a knife, and the victim was stabbed in the chest. Police say the knife pierced the victim's heart.

One 27-year-old suspect is in custody and faces charges of attempted murder and aggravated assault. Police are still looking for the second man, who they believe used the knife.

Police describe him as white or Middle Eastern, with dark short hair and a chinstrap beard. He was wearing a zebra pattern sweater.

Police say the man may be familiar to people in the area of Yonge-Dundas Square.

Anyone who sees the man is advised to call police.