Waiters locked themselves inside the restaurant as two groups clashed outside the Hanging Gardens Restaurant, hurling furniture across the footpath.
One staff member said he and his colleagues stayed inside during the fight, which lasted for between five and 10 minutes.
The group had left a nearby bar about 11.30pm, after celebrating an 18th birthday .
Managers of the bar told Seven there had not been any trouble during the gathering.
The fight began as the teenagers left the bar, next to the restaurant.
Video footage shows a number of men and women punching and kicking each other and throwing furniture, as others try to break up the fracas.
Lygon Street trader Agron Tairi said the brawl was “a little bit overwhelming”.
“But there’s not much you can do, really,” he said.
A police spokeswoman confirmed police received reports of a brawl between 10 and 20 people about 11.35pm on Lygon Street.
“Local police attended a short time later and found the fight had finished,” she said.
“Police have not received any complaints about the incident.”
An Ambulance Victoria spokeswoman said two teenage boys were assessed by paramedics shortly after 11.30pm.
She said both boys had suffered upper body injuries in the fight and were taken to Royal Melbourne Hospital. Both were in a stable condition, she said.