Nineteen people have been injured, several of them critically, after pedestrians were mowed down by a car in peak hour in Melbourne's CBD in what police say was a "deliberate act".
A white Suzuki SUV showed no sign of slowing as it hit people at the busy intersection of Flinders and Elizabeth streets about 4.40pm, including some waiting at a tram stop.
The 32-year-old driver was arrested by an off-duty police officer shortly after the car crashed into a metal bollard on Flinders Street.
Police said he is an Australian citizen of Afghan descent.
The male officer was followed by general duties police and members from the Critical Incident Response Team.
Witness Chris Gath said he saw 60 to 100 people crossing the intersection when a car travelling at up to 70km/h headed for the crowd.
Several bystanders had earlier stepped in to restrain one of the men in the car.
A second man, 24, was arrested shortly after but police do not believe he was involved in the attack.
It's now believed he had no links to the incident, however he is still assisting police with enquiries.
Witness Lachlan Vella said he saw a man in a suit lying on the floor surrounded by people.
"His head was busted open".
Nearby was a woman who was "coming back to consciousness and asking what was happening", and another man "who had blood around his head and was snoring, like atonal breathing."
Soon after, he saw police arrive at the scene and pulled out his phone to film the arrest.
The clip that he posted on social media would become the most vivid footage of the arrest to be circulated on Thursday.
Visibly shaken, Mr Vella recalled seeing a man in blue jeans and a shirt inside the car.
Bystanders were inside of the car, holding the man, he said.
He then saw the man get pulled from the car by police officers, one "with a giant gun at his chest ... like the big one".
His footage shows three officers holding down a bearded man in a white T-shirt and light blue jeans moments after he was pulled from the car. Officers cuff the man while he is face down then turn him over.The man appears to be unconscious and is slapped in the face by an officer. He then appears to regain consciousness.
Police commander Russell Barrett did not rule out terrorism as a possible motive.
"We believe based on what we've seen it is a deliberate act," he said. "It is too early in the investigation to discuss motivation.
"Police will continue to have a strong presence within Melbourne's CBD tonight," he said. "People will not be able to retrieve items left in the crime scene."Earlier, an officer at the scene described it as a terrorist incident, but this was not confirmed.
It appears police did not enact counter-terrorism lockdown strategies. The new terror warning sirens were not activated.
People were struck in front of dozens of others as they crossed the intersection at the start of peak hour.
Police and emergency services remain on the scene and will do for the foreseeable future.
The crime scene is active and will remain in place until at least 7am tomorrow.
Police urge people who can avoid the area, to avoid the area.
Child among injured
A preschool-aged child is among the injured and has been taken to the Royal Children's Hospital in a serious condition with head injuries.Thirteen people were taken to hospital, including three to The Alfred and three to the Royal Melbourne. Their injuries and condition are not yet known.
Police said investigators were still piecing together how the incident occurred. They called for witnesses to immediately come forward.
"Police have saturated the CBD area in the vicinity of Flinders, Elizabeth and Swanston streets to ensure community safety," police spokeswoman Creina O'Grady said.
Public transport affected
Sections of the city's public transport system were in lockdown, with Metro Trains urging commuters to avoid Flinders Street.
"Due to a major incident at the intersection of Elizabeth and Flinders streets, passengers are advised to defer travel through Flinders Street Station," Metro Trains tweeted. "Please interchange for services at Southern Cross and City Loop Stations."
Yarra Trams cancelled services to the area.
A huge area of the city was cordoned off as crowds tried to navigate their way home.Police progressively pushed people back from the site of the incident.
Whole streets were clogged with police cars, ambulances and other emergency vehicles.
By about 8pm emergency services had started to vacate the immediate area of the mayhem, although a huge part of the city remained cordoned off to pedestrians and cars.
'People are flying everywhere'
Sue from Walker's Doughnuts on Elizabeth Street told radio station 3AW that she heard screams before she saw "people flying everywhere"."We could hear this noise. As we looked left, we saw this white car, it just mowed everybody down," she said. "People are flying everywhere. We heard thump, thump. People are running everywhere."
A young mother who was at the scene with her young daughters told radio station 3AW they fled as the car ploughed into pedestrians."I didn't want the kids to see anything," she said. "We just ran.
"I wanted to get the girls out of there so we ran into the closest building.
Chris Gath said he was standing at the 7-Eleven on the corner of Flinders and Elizabeth streets with a coffee in hand when he heard screaming.
"I heard it first and then I turned around and saw lots of bodies on the floor.
"I saw a car ploughing into many people and bodies flying everywhere."
He marshalled a group of people, including women with prams, away from the intersection to safety.
"I was a bit worried about myself so I got lots of people to run with me, and got families out of the way.
"I didn't know what was coming or how organised it was, whether it was a bomb... so I just freaked and ran.
Rohit Kaul, 45, was on the phone to his wife, Avi, and crossing the intersection at Elizabeth Street when he was hit by the car.Ms Kaul heard the loud noises down the line before her husband's phone cut out and she unsuccessfully tried calling him several times.Mr Kaul sustained leg injuries and was helped by several pedestrians who had been walking in front of him. He managed to return his wife's call and was taken to The Alfred hospital.
Hundreds of people milled around the edges of the crash scene, with police quelling a group in the crowd calling out that the incident was a terrorist attack.
Footage from the scene showed crowds of scattered people surrounding the white SUV in the middle of the intersection.
Police asked any witnesses to go to the Melbourne West police station at 313 Spencer Street, Melbourne. About 20 people entered the police station within half an hour of their request. Some wiped away tears while others buried themselves in the arms of loved ones.
Among them was Kat Edwards, who said she had been at the tram stop near the intersection when she heard the vehicle.
"I saw the driver, but I also saw a man lying at a tram stop, so I decided to run towards that man and tend to him."
Prime Minister Malcolm Turnbull and Premier Daniel Andrews tweeted statements of support for the victims and emergency service workers on the scene.
"As our federal and state police and security agencies work together to secure the scene and investigate this shocking incident, our thoughts and prayers are with the victims and the emergency and health workers who are treating them," Mr Turnbull tweeted.
Earlier, Mr Andrews warned Melburnians to avoid the area. "Stay safe. Check on your loved ones. And thank you to our brave emergency services."
Members of the public who have video or images of the incident are asked to upload them here: http://australiauploadserviceweb.cloudapp.net/