Eleven officers injured in a drugs raid in north London, were "all armed police", the Met Police has said.
They were all injured by a substance, believed to be sulphuric acid, in Barnet on Wednesday afternoon but have all been discharged from hospital.
Cdr Kyle Gordon said they had shown "immense bravery".
Four men were all arrested on suspicion of Class B drug offences and causing grievous bodily harm against the officers.
A 23-year-old man and a 30-year-old man are still both in custody, the Met added.
But a 21-year-old suspect remains in hospital for treatment to burn injuries while a 32-year-old is also in hospital recovering from a foot injury.
The Independent Office for Police Conduct (IOPC) is investigating how the man came to suffer the foot injury during the arrest.
All 11 officers had been called to an industrial area in Dale Close at about 13:50 BST.
Cdr Gordon added: "Thankfully, none of the 11 officers were seriously injured and they should all make a full recovery.
"We will continue to support them in any way they need, as they return to their work of keeping the capital safe."