Police in Australia say a suspect who they think deliberately drove a car into a crowd in Melbourne has a history of mental health problems.

19 people were injured, four critically.

Both the suspect and the off-duty police officer who arrested him are among those being treated.

Officers say the main suspect is a 32-year-old Australian national of Afghan descent who has a history of drug use & mental health issues.

Police say are still trying to work out any motivation behind the incident.

Shane Patton, from Victoria Police, added: "We don't have any evidence or intelligence to indicate a connection to terrorism.

"Having said that, however, we continue to support this investigation with our counter-terrorism command to ensure that there isn't that connection and there is no ongoing threat."

Media conference regarding the Flinders Street incident with Commander Russell Barrett. pic.twitter.com/j7zx0V5fYQ

— Victoria Police (@VictoriaPolice) December 21, 2017

Australian Prime Minister Malcolm Turnbull responded to the incident on Twitter: 

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

A second man was also arrested at the scene in Melbourne. He was seen filming the incident, with police saying he was found carrying a bag containing knives.

Police said: "It’s now believed he had no links to the incident, however he is still assisting police with enquiries."