Australian police said terrorism was likely not a factor in Thursday's vehicle attack on a busy pedestrian crossing in central Melbourne that left 19 people injured, some critically.

Victoria Police Acting Chief Commissioner Shane Patton said a 32-year-old man arrested after deliberately ramming a white Suzuki SUV into pedestrians on Flinders Street had drug and mental health problems.

He confirmed that 19 people were admitted to hospital after the attack, which occurred at 4:41 p.m. local time. Four people are in critical condition while the remaining 15, including the driver, are stable.

Speaking at a press conference, Patton said the driver, an Australian citizen of Afghan descent who was driving alone, was arrested at the scene by an off-duty police officer.

A 24-year-old man was also arrested at the scene after being seen recording the incident on his mobile phone. The police found three knives in his possession, however Patton said it was "quite probable" he was not involved.

"There may be no relationship between (the driver and the 24 year-old). But obviously, at a significant scene like this, he was taken into custody because of the circumstances so we can explore it," he said.

The police said the driver was known to Victorian state police for historical assault charges, but is not currently under any correctional orders.

"He has a history of drug use and mental health issues," Patton said. "This is early days, but we understand he is on a mental health plan and is receiving treatment for mental illness."

Several police squads, including the counter-terrorism squad are assisting in the investigation only as a precaution.

"At this time we do not have any evidence or intelligence to indicate a connection with terrorism," Patton said.

A similar incident occurred in January when a car sped down a nearby pedestrian mall in Bourke Street, killing six people and injuring 30 others. Among the dead was a Japanese student.

Although the January incident was not deemed a terrorist act, concrete barriers have subsequently been placed at entrances to pedestrian areas in major cities around Australia, including Melbourne.

Further measures to protect the Melbourne community were announced by the Victorian state government less than two weeks ago with the installation of loudspeakers at more than 90 locations around the central business district to provide warnings and key information during emergencies.

A trial demonstration of the alert system is due to be held on Dec. 28.

Assistant Commissioner Ross Guenther, who is in charge of counterterrorism efforts at Victoria Police, told The Australian newspaper early this month that Christmas is the busiest time of year for counter-terrorism squads.

"Ideologically, the markers for us, for the Christian events, would certainly be things like carols by candlelight, Christmas Eve, New Year's Eve, events we have at this time of year where crowded places are very active," he told the newspaper.