Man dies after being arrested in Sydney

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 a street in Newtown after a man, who was being held under the Mental Health Act, fled a nearby hospital, NSW Police said in a statement on Sunday.

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

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 the Seven Network.

"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.