The man who deliberately drove a car into a crowd in Melbourne is a drug user with mental health issues but no known terrorism links, police say.
The Australian citizen of Afghan descent was taken into custody after a struggle at the scene of the incident.
The car the 32-year-old was driving hit a number of pedestrians on Flinders Street, a busy thoroughfare in the city centre, said Victoria Police.
Fourteen people have been injured, with several in a critical condition.
A second man, 24, was arrested after being seen filming the incident. He had a bag of knives with him, police added.
"It is now believed he had no links to the incident, however he is still assisting police with inquiries," they said.
The driver was arrested by an off-duty police officer. Police say he was the only person in the car.
The officer sustained shoulder and hand injuries during the arrest and was taken to hospital.
The driver was also taken to hospital, under police guard.
The white SUV struck pedestrians just after 16:30 local time (05:30 GMT).
Witness Jim Stoupas, who runs a business nearby, told the BBC: "It just barrelled through a completely full intersection of pedestrians. There was no attempt to brake, no attempt to swerve."
He added: "I saw probably five to eight people on the ground with people swarming around them [to help]. Within a minute, I think, there were police on site, so it was very, very speedy."
Ambulance Victoria said in a statement that a child of pre-school age with serious head injuries was among those taken to hospital.
Another witness, Lachlan Read, told the Herald Sun the whole incident lasted about 15 seconds.
"He has gone straight through the red light at pace and it was bang, bang, bang. It was just one after the other," he said of the moment the vehicle started hitting people.
Rossella Belardi told the BBC she was coming out of Flinders Street Station when she saw people running. "Many people were on the floor and smoke was coming out of the car."
"Police and the ambulance service were incredible," she added. "They came immediately out of nowhere."
Prime Minister Malcolm Turnbull said on Twitter that the investigations had begun, and sent "thoughts and prayers" to those affected.
As our federal & state police & security agencies work together to secure the scene and investigate this shocking incident our thoughts & prayers are with the victims & the emergency & health workers who are treating them. https://t.co/JkUM9TuIQe— Malcolm Turnbull (@TurnbullMalcolm) December 21, 2017
The leader of the Australian opposition, Bill Shorten, also tweeted about the "shocking scenes" and praised the emergency services.
Shocking scenes in Melbourne this afternoon. Credit to first responders who are doing us proud once again. Thinking of everyone caught up in this atrocity.— Bill Shorten (@billshortenmp) December 21, 2017
In January, six people died when a man drove a car into pedestrians on Bourke Street.
Afterwards, city authorities installed concrete blocks in various locations - including on Flinders Street - hoping to prevent vehicle-based attacks.
In September, a 15-year-old boy dressed in black combat gear was seen driving erratically down nearby Swanston Street.
After a confrontation with police he was subdued with a Taser outside Flinders Street Station. Police later said it was not a terrorist incident.