A woman was badly injured when she was crushed between two cars in a possible road-rage attack in central London.
The Met said officers were looking into claims a Land Rover was driven at the woman, trapping her against a Mercedes, near Hyde Park Corner on Sunday night.
The woman, in her 40s, was taken to hospital where her injuries were described as possibly life-changing.
A 23-year-old man was Tasered and arrested at the scene on suspicion of affray.
He has subsequently also been arrested on suspicion of grievous bodily harm and driving while disqualified in connection with the incident at about 20:15 BST, the police force said.
Tourist Deepak Anand, 40, from Vancouver, Canada, was on a bus with his wife and son and filmed what happened at the junction of Grosvenor Place.
Mr Anand said he saw a man trying to pull a driver out of a Land Rover, which then "accelerated on to oncoming traffic" before lodging the woman between the car and a black Mercedes-Benz.
The Mercedes also hit a stationary bus but there were no reports any passengers were injured.
Mr Anand said police officers arrived shortly after and Tasered a man.
Witnesses said the woman could be heard screaming and fell to the ground after the cars were separated.
Police said the woman had been taken to a central London hospital for treatment.
A Metropolitan Police spokesman said: "At this early stage it is believed she was the occupant of a car who became involved in a dispute with another car.
"She had got out of her car before being struck by the other car as it attempted to drive away."
A second man, who complained of feeling unwell at the scene, was taken to hospital and has since been discharged.