Two teenagers - a man and a woman - were also arrested.

Police were called to Pitt Street about 4am, following reports of two assaults, to find more than 20 men and women fighting outside a convenience store.

A fight reportedly began inside the Arthouse Hotel, which left a man unconscious with serious head injuries, Channel Nine reported.

A violent brawl also erupted outside the hotel, with a crowd of up to 40 spilling onto the street.

More than a dozen police attended, using pepper spray to disperse the crowd.

Police said a 19-year-old woman was treated at the scene for the effects of pepper spray and was taken to St Vincent's Hospital for assessment.

Two men, aged 18 and 29, and a woman, aged 19, were arrested at the scene.

The men were taken to Day Street police station, while the woman was taken to Surry Hills police station.

The teenagers were later released, pending further inquiries.

The 29-year-old man, from South Hurstville, was charged with affray and resisting arrest. He has also been banned from licensed premises in Sydney's CBD.

The 18-year-old man released by the police said "the other people" had started the fight.

Asked about her injuries, a female companion walking behind him said: "Yeah, I'm all right, but."

Police said they were continuing their inquiries and were speaking to a number of witnesses.

Seven News reported that police were also investigating whether security guards were involved in the fight.