How an off-duty policeman became a hero after yesterday's SUV attack in Melbourne.

Yesterday, as a driver ploughed a car through pedestrians in Melbourne, most onlookers stood frozen in shock and panic.  Their shopping bags and ice creams and phone conversations immediately forgotten in fear.

“We didn’t know what was going on,” one witness told the ABC. There were people injured, bleeding on the pavement. Others were running, desperately trying to escape the scene.

But one man, who’d been hit and injured by the rogue vehicle, did something completely different.

He was hurt and scared and without any backup, but he was also an off-duty police officer. And, without worrying about his injuries or the potential imminent danger, he descended on and arrested the alleged man behind the wheel.

Now, as he lies in hospital alongside 18 other wounded, this off-duty police officer (who has remained unidentified) is being hailed a hero by the public and his colleagues alike.

"To step in when you see that trauma, not understanding what the threat is, and take somebody into custody not knowing what he is risking himself — it is heroic," Acting Chief Commissioner Shane Patton told the ABC.

Remembering that only moments before he "stepped in", he too had been struck down by the white vehicle travelling too fast by a driver who, police say, was deliberately aiming to cause harm.

Off-duty cop praised for rampaging Melbourne driver arrest https://t.co/rUdKbF7Pa4

— #aupol news (@AupolNews) December 21, 2017

'Heroic' off duty police officer praised for arresting driver of Melbourne car attack https://t.co/XwTPBN2q9i pic.twitter.com/RpMXO4WdTi

— HuffPost UK (@HuffPostUK) December 21, 2017

Tonight our thoughts and prayers are with the victims and families of the terrible incident in Melbourne. May their recovery be swift and their trials eased.

We also commend the courage of the off-duty Police Officer whose actions likely saved more people from injury. 1/2

— MuslimsAustralia (@MuslimsAus) December 21, 2017

The alleged 32-year-old driver, who is an Australian citizen of Afghan descent, is among the 19 injured.

Speaking to the press, Patton said the arrested man was known to police, was partaking in a mental health plan and had drug abuse issues. He was not on bail at the time of his arrest, AAP reports.

"He is still in custody, under arrest for these offences, for what we allege was a deliberate act," Mr Patton told reporters on Thursday night, as reported by the ABC.

A four-year-old boy with head injuries is among the victims, while four people remain in a critical condition, AAP reports.

It's not yet known if the act was one of terror, but the timing couldn't be more harrowing.

The mall was busy with people searching for Christmas presents and enjoying the holidays. Only for a happy, warm, summer's day to turn to tragedy as the white SUV barrelled down the pedestrian crossing.

"It hit people in a fair speed and at least one or two of them got thrown pretty high into the air," one witness David told the ABC.

Another witness, Jim, said "the only thing that slowed him down was him hitting pedestrians".

"There was no braking, there was no slowing down, he went straight through the intersection. All you could hear was the sound of the car hitting people and screams."

Victoria's Premier Daniel Andrews has labelled the incident an "act of evil and an act of cowardice".