Sergeant Martin Flannery said a member of the public flagged down a divisional van shortly after midnight, asking police to assist a man who looked like he was suffering a seizure near the intersection of Percy and Tyers streets.

While the two officers were providing first aid and waiting for an ambulance to arrive, a drunk man aged in his early 40s approached the police and became aggressive.

Officers warned the man to stand back and not interfere but he continued to behave in an aggressive and threatening manner.

A police officer was forced to use OC capsicum foam to subdue the man and ensure the safety of the police and members of the public.

The man was not arrested and left the area while police continued to provide first aid.

Sergeant Flannery said the man was known to police and would be interviewed.

"I would expect he will be charged on summons with offences which may include hindering police and behaving in a riotous manner," he said.

"This is not the first time we've encountered this sort of situation. Police would ask that people stay back and allow police to do their job because as a passerby you may not know the full story of what is going on."

Sergeant Flannery said it was difficult for police to do their job while dealing with another drunk and aggressive person.

He said one of the police officers was also affected by the OC capsicum foam.

The 27-year-old man suspected of having a seizure was transported by ambulance to hospital.

He was later diagnosed as simply being intoxicated and has been discharged from hospital.