A man was killed and another injured after they came to the aid of a woman struggling with a gunman in Australia's second-largest city, police say.
The dead man is believed to be a 43-year-old solicitor
The injured man and the woman are said to be in a critical condition after being shot during the morning rush hour in Melbourne's business district.
The area was cordoned off, with police evacuating office buildings as a hunt for the gunman got under way.
Police said they had identified the suspect they were looking for.
They described him as a Caucasian male in his late 20s or early 30s, who was known to them.
A handgun and denim jacket believed to belong to the gunman were recovered from a construction site, but police warned that he could still be armed.
The incident on the corner of Flinders Lane and William Street happened as people were arriving for work around 0800 local time (2200 GMT Sunday), sending hundreds of commuters fleeing for cover.
"A girl came out of a building over the road - she was screaming and a guy had her by the hair," witness Ross Murchie told the Australian Broadcasting Corporation.
"She tried to grab hold of a taxi that was going by, and the couple of bystanders went over to ask what was happening," Mr Murchie said.
"He let go of her hair, pulled out a gun and shot them all."
The two men who came to the aid of the woman were described by the police and media as "good Samaritans".
Detective Inspector Stephen Clark said the man who died, reported to be a 43-year-old solicitor, made his last act "one of extreme bravery".
"It's a tragic, tragic set of circumstances and it does appear as though he's been in the wrong place at the wrong time and has gone to the assistance of a female," he said.
The other man, believed to be in 30s, and the 24-year-old woman were both said to be in a critical condition in hospital after receiving surgery for gunshot wounds to the chest.
Witnesses said there had been an altercation with the gunman and a woman in a nearby nightclub shortly before the incident.
Police Inspector Glen Weir said the shooting incident did not appear to have been "a random act".
The incident forced the closure of a number of streets in the commercial heart of Melbourne, with shops and offices shut as armed police and helicopters began the hunt for the gunman.
Police began escorting workers back to their offices after a couple of hours.
Were you affected by the incident? Did you witness the shooting? Do you know any of those involved?
I am a Uni student and usually go through Flinders Street Station to get to uni, today was absolute chaos, cops everywhere. I thought they were filming a movie! Cops were doing rounds with guard dogs everywhere, with loaded shotguns and semi-automatic rifles. Quite a scary scene, once you realise it's for real and not a set-up.
Nermin Mujcinovic, Melbourne, Australia
Our tram stopped on the corner of Flinders and Queens St, and we walked up the street past the scene. We saw one body on a stretcher with a sheet draped over it, and scores of paramedics furiously working on the other two victims. At first we thought it was simply a terrible road traffic accident, but a few minutes later a policewoman in a flak jacket asked us to move away from the area, as it was still unsafe. Once at work on Lonsdale St, we were told by our company to remain indoors for several hours until police effectively gave the all clear.
Martin Hammarberg, Melbourne, Australia
I work two blocks from the shooting and saw the man's body covered with a sheet as I was driving into work along Flinders Street. It usually takes me about 50 minutes to drive into the CBD but today it took me close to 1 hour and 30 minutes. When I finally got to work, I parked in the building's basement car park and was told there were Victoria Police SWAT teams checking our basement car park in case the gunman was hiding there. Along with all the police and helicopters in the sky it seemed like a scene from a movie. I later found out that the gunman tossed the gun into a construction site across the road from my building. It is a scary thought that a maniac like that is running lose in the city.
John, Melbourne, Australia
I saw the whole thing from a tram going around the corner. It was absolutely a horrific thing to see. I saw him shoot the woman and the first man who died soon after. He was trying to help the lady who was being grabbed by the hair from the gunman. He has paid for his good deed with his life. I can't believe that this has happened here in Melbourne, I am still very shaken almost 11 hours after the whole experience.
Anon, Melbourne Australia
I am currently a geordie boy working in melbourne and the whole city was in chaos this morning I work on flinders street and the whole place was locked down, trams cancelled, people locked in buildings, a real nightmare. The city is getting back to normal this evening.
Paul Dixon, Melbourne
I was on my way to the office at the Rialto Towers at 8.30 this morning, when I arrived the building had been cordoned off by the police. I heard the rumour of a gun man was on the loose and they were searching our building for him. They shut down all 53 floors of the building with the thought he might of ran inside. There were police everywhere and every body was very concerned. When I finally got into the office an hour and a half later, I could look down on the streets below and see a body under a sheet on the road. It was horrific.
Matt Davies, Melbourne, Australia
I am an english backpacker on my way to work when all the mayhem was going on, there were helicopters surveying the area! i thought it was all part of a film. Streets were closed and people were being told to stay indoors!
Chris Partridge, Melbourne
I used to work exactly where it happened. Its crazy. Melbourne is one of the safest cities I've ever been to, you can easily walk round on your own at 1am and never have a problem so for something like this to happen is mad. I can't believe I used to walk along that street every day.
Kathy, Newcastle, England