Northumbria Police is appealing for witnesses following the incident, which happened at around 4.25am on Saturday, when officers received a report of an assault at the taxi rank on Anderson Street, opposite Can Can Bar.
A force spokesman said: "It is believed that an unknown man attacked a 28-year-old man, causing him to fall to the floor unconscious.
"The victim was subsequently taken to hospital with a broken jaw, broken tooth and head injuries.
"Inquiries are ongoing to determine the circumstances behind the incident, and police are now urging anybody who was in the area at the time and saw the assault to get in touch."
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Anyone with information is asked to contact police on 101 quoting reference 215 180818 or Crimestoppers anonymously on 0800 555 111.