Scotland Yard is treating the incident as a terror attack.
Police have revealed that there have been “a number of casualties” with much of London now in lock-down, and the terror-alert level raised to the highest level.
Ambulances have treated over 20 people on Westminster Bridge, with another woman being pulled alive for the River Thames after being hit by the car on the bridge.
Leader of the Opposition, Bill Shorten has taken to Twitter to offer support of the UK.
“This shocking crime, designed to weaken the U.K., will only make her stronger. All Australians stand with Britain today,” he wrote on the social network.
Those killed include a police officer and the attacker, a woman and another person were both killed when struck by the car on the bridge.
Australian Foreign Affairs Minister, Julie Bishop, has said that at this stage no Australians have been involved in the attack.
"It certainly has Hallmark hallmarks of some of the recent Isis inspired terrorist attacks," Bishop said in a statement.
"It is too early for a conclusion to be made on that way."
The attacker has not yet been identified.