Injured police officer chased alleged attacker through Sydney CBD before arrest

A man has been arrested after allegedly stabbing a police officer in the back of the head in Sydney's CBD.

Police said a 33-year-old man approached the officer at the intersection of Park Street and Castlereagh Street at about 1pm before allegedly injuring him with a knife several times.

The police officer and a colleague chased the alleged attacker through nearby Hyde Park, where the man was Tasered and arrested.

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In footage of the tense standoff before the man's arrest on Elizabeth Street, police can be heard warning the man to "put the knife down".

The officer was treated by paramedics at the scene and taken to St Vincent's Hospital for treatment for non-life-threatening injuries.

The 33-year-old man was arrested and taken to Royal Prince Alfred Hospital for further assessment.

He has not been charged with any offences.

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NSW Police Superintendent Martin Fileman said the man "didn't say anything" before he walked up to the officers.

"There was no conversation," he said.

Fileman said the alleged stabbing was an isolated incident but the man had been known to police for several minor matters.

"There is no history of mental illness on our database," he said.

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A large police presence was seen on Elizabeth Street at Hyde Park near Bathurst Street.

NSW Police said there is no ongoing threat to the public.

A second crime scene was established at the intersection of nearby Castlereagh Street and Park Street.

Several roads had been closed in the city due to the police operation, but have since reopened.

The southbound bus lane on Elizabeth Street remains closed.