Police are investigating after a teenage boy was stabbed in the stomach during a fight in inner Sydney overnight.

The 16-year-old was walking with friends on Lee Street in Haymarket, near Central Railway Station, when they got into a fight with another group about 11pm (AEDT).

The boy was stabbed once in the stomach.

Acting Inspector Steven Warren says police believe the groups are not known to each other, and it is unclear what the fight was about.

"It appears that they just came together on the street and it became violent," he said.

The teenager was taken to St Vincent's Hospital but police say his injury is not life threatening.

No-one else required medical treatment after the fight.

Police arrested three youths at the scene.

They were questioned at Surry Hills Police Station but were then released without charge.

"There was an issue with timeliness. After midnight we couldn't get Legal Aid to give them advice," Acting Inspector Warren said.

"They've been released into the custody of their parents and they'll come back at a later date for further questioning."