'Chaos' after man shot, four injured in separate incidents on same Melbourne bridge at same time

A man has been shot and four have been injured in two separate incidents after a night of "chaos" in Melbourne's CBD.

The first incident unfolded inside a tram when a man allegedly tried to rob someone before exiting onto Princess Bridge at 5.30 pm.

Victoria Police allege the man then assaulted four people with a glass bottle, including a 67-year-old man, a 56-year-old woman, a 24-year-old woman and a 46-year-old woman.

They were all taken to hospital with non-life-threatening injuries.

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Witness Nick Alonso told 9news.com.au he was on the tram going past the art precinct when he saw a big group of people.

"One of the women there had a big gash on her face and she was bleeding," Alonso said.

He said he continued on the tram to Flinders Street Station where he saw police arrive and arrest a man.

"They handcuffed him and he was put onto the floor. It was chaos."

The 27-year-old man was arrested and charged with four counts of recklessly causing injury.

A second incident broke out at the same time when a second man began threatening protective services officers at Princess Bridge.

Victoria Police say the man was armed with broken glass when he confronted the PSOs.

"Two PSOs have deployed OC spray, which didn't appear to have any affect," police said.

"One PSO discharged his firearm, with the man being struck once."

The PSOs were responding to the other incident when the confrontation took place.

The injured man was taken to hospital with what are believed to be non-life-threatening injuries.

Police said the incident was not terror-related and there was no danger to the community.

Armed Crime Squad detectives will investigate, with oversight by the Professional Standards Command, as per standard protocol when a firearm is discharged.