What happened

The streets outside Melbourne's iconic Flinders Street Station was busy with commuters and shoppers Thursday when witnesses said the SUV plowed into people crossing the street.

Witnesses told CNN affiliate Seven Network they saw people flung into the air after being hit by the car, which was barreling down the road at around 60 miles per hour (100 kph).

Police arrived on the scene within 15 seconds and the man was taken into custody by an off-duty police officer after attempting to resist arrest, Patton said. Both the man and the police officer were injured in the scuffle.

A 24-year-old man was also arrested at the scene, after he was seen filming the incident on his phone. Police found three knives in his bag. The man is being interviewed by police but Patton said at this stage, "we don't yet have any relationship established between the 24-year-old man and the driver... in fact, there may be no relationship between them."

The driver was known to Victorian Police due to "historical assault matters" and a history of drug use. "We understand he is on a mental health plan and receiving treatment for a mental illness," Patton said.

Victorian Premier Daniel Andrews called it a "terrible, evil cowardly act," but said there was no need to change the state's threat level. He urged Victorians to go about their business as usual and enjoy the holiday season.

Flinders Street is one of Melbourne's busiest pedestrian roads, running between a major train station and the bustling city center. It would have been crowded with commuters and shoppers making their final purchases in the days before Christmas.

Paramedics took 19 people to nearby hospitals, including the driver. Two others were treated at the scene. Victoria Police spokesman Comm. Russell Barrett told reporters several of those injured were in a critical condition.

Among them was a pre-school age child who was taken to hospital with a head injury, and is in a serious condition, Ambulance Victoria said.

'It just plowed into the crowds'

Jim Stoupas, owner of the Walkers Donut shop on Elizabeth Street, told CNN he saw an SUV going about 60 miles an hour (100 kph) down Flinders Street.

"It just plowed into the crowds," he said. "We heard this noise and we looked up then we just heard bang, bang, bang and people flying everywhere. We had a lot of customers in our shop and they were traumatized."

Another witness, Cameron Von Borstel, said he assumed a car had crashed into a tram when he heard a loud bang from the direction of Flinders Street.

"I went down the street to have a look. I saw about two or three people lying still in the street. People were helping," he told CNN.

The streets and landmarks around Melbourne's Flinders Street station have been the target of multiple attempted terrorist plots, all foiled by police within the past year.

It was described as "one of the most substantial terrorist plots that have been disrupted over the last several years."

'Shocking scenes'

Plans for a public memorial to mark that event were announced this week by Victorian Premier Andrews.