A 59-year-old man suffered a traumatic brain injury when an argument with a younger man turned violent at Hallidie Plaza late Wednesday, police said.

The assault happened about 20 minutes before midnight when the suspect, a 29-year-old man, started to attack the older man. Police said the victim was knocked out during tha altercation.

The victim was admitted to St. Francis Memorial Hospital later that night and is expected to survive.

The assailant used no weapons and has not been arrested. No detailed description of him has been released.

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