Melbourne - A car ploughed into pedestrians on a popular shopping strip in the heart of Australia's second-largest city on Friday, killing four people including a young child and injuring dozens more.
Police said they were not considering the incident an act of terrorism.
Witnesses described "bodies flying through the air" and people running out of the path of the speeding vehicle in the central mall area of Melbourne.
Moments before, passers-by had watched astonished as the driver gesticulated out the window and spun the maroon-coloured saloon round a major intersection, blocking traffic.
The trail of violence took place barely a kilometre (half a mile) away from Melbourne Park where the world's top tennis stars were playing the opening Grand Slam of the year in a city teeming with tourists.
Police shot the 26-year-old male driver - who has a history of family violence, mental health and drug issues - in the arm and arrested him.
"This individual is not related to any counter-terrorism or any terrorism-related activities," said Graham Ashton, chief commissioner for the state of Victoria.
"He is not on our books as having any connection with terrorism and we're not regarding this as a terrorism-related incident.
"What we do know of the person is there is an extensive family violence history involved." Three people were killed at different locations on Bourke Street, a man and a woman in their 30s and a child whose age was not given for privacy reasons, Ashton said.
A fourth person was confirmed dead by police late Friday, but their age and gender were not released.
Fifteen others were hurt. Five were in a critical condition, including an infant, he added.
The Royal Children's Hospital said it was treating three children aged three months, two years and nine years. A fourth child aged 13 was treated and discharged late Friday.
"The prayers and heartfelt sympathies of all Australians are with the victims and the families of the victims of this shocking crime in Melbourne today," Prime Minister Malcolm Turnbull said in a statement.
The driver allegedly stabbed his brother in the suburb of Windsor six kilometres south of Bourke Street early on Friday, Ashton said.
Police had tried to intercept his car before it entered the city about 1.45 pm local time (0245 GMT) and he was seen driving erratically outside Melbourne's main train station in the afternoon.
"We believe this male did some 'doughnuts' at Flinders Street and Swanston Street, turned left into the Bourke Street mall and deliberately drove into the crowd, continued along the footpath colliding with further pedestrians," Police Acting Commander Stuart Bateson said.
YouTube footage showed the driver wearing a light-coloured top half-hanging out of the vehicle and gesturing to pedestrians.
One witness, Kylie Clark, told Melbourne's Herald Sun she saw "bodies flying through the air".
"It seemed to me they (the vehicle that hit people) were being chased by an unmarked police car." Another witness told the newspaper "he wasn't stopping and people walking were trying to get away and he just kept going and collected people on his way".
A witness, who asked not to be named, told AFP "it was carnage up the footpath".
Heavily armed officers were deployed around the scene as television footage showed the badly damaged car with the front doors open and the windscreen smashed.
A handcuffed man in his underwear was seen lying on a footpath surrounded by police.
Other pictures showed an upturned baby pram. The child in the pram was one of the critically injured, Aston said.
The suspect was charged last weekend by police, Aston added, without giving further details.
"He has come to our attention on many occasions in the past. We have mental health and drug-related issues in the background of this particular person.
"He has been coming to our attention more recently over recent days in relation to assaults, family violence-related assaults."