Eleven Met Police officers have been injured by a "corrosive substance" during a drugs raid in north London.
They had been called to an industrial area in Dale Close, Barnet, at about 13:50 BST.
All of the injured officers have been taken to hospital, but are not thought to be in a life-threatening condition, the Met said.
A number of males were arrested on suspicion of drugs offences.
The London Ambulance Service (LAS) said four people other than the officers were also treated at the scene.
A LAS spokeswoman said 15 people in total were taken to hospital for injuries - it is not thought any are life-threatening at this stage.
She added: "We dispatched a number of resources including ambulance crews, two incident response officers, an advanced paramedic and medics in fast response cars.
"We also dispatched our hazardous area response team (HART). "
The Met said it was in the process of informing the injured officers' families. Other officers remain at the scene.
The force's Directorate of Professional Standards had been informed, as was routine, it added.
Reacting to the injuries Mayor of London Sadiq Khan said he was "shocked" by the news.
"It is a stark reminder of the real dangers our hardworking officers face every day as they keep us safe," Mr Khan said.
"On behalf of all Londoners, I wish them a speedy recovery."