Man dies during arrest in Newtown

A 30-year-old man has died after he collapsed while being arrested in Sydney's inner west, sparking a police internal investigation.

Officers were called to Carillon Avenue in Newtown after a man, who was being held under the Mental Health Act, fled the nearby Royal Prince Alfred Hospital hospital.

"The man resisted arrest and collapsed at the scene before being taken to hospital, where he later died," the statement said.

There are reports a Taser was used in the arrest.

A witness said she saw 15 officers "on top of him" during the arrest.

"My gut feeling was that maybe they used a bit of excessive force trying to arrest him," she told reporters.

"He looked like he may have been a big man but still, and they pulled him back up which made it even worse."

The investigation will be independently reviewed and a report will be prepared for coroner.