Three men have been arrested following a mass brawl outside Sydney's Town Hall overnight.

Officers arrived about midnight to find a group of men fighting in George Street, as bystanders looked on.

Acting Inspector Chris Breed of Sydney City Local Area Command said that about 20 people were involved.

Police arrested a 16-year-old youth and two 20-year-old males at the scene. All three were taken to Day Street police station, where they were later released pending further inquiries.