Two men and a 17-year-old boy have been sprayed with what police believe to be a "noxious substance" following an argument between two groups of men.
The men aged 22 and 27 and the boy were injured following a "large altercation" in Shacklewell Lane in Hackney, London at about 05:20 BST.
They suffered non-life threatening injuries but others may have been hurt.
The Met is investigating the substance but officers believe it was noxious and not a corrosive one such as acid.
It is still to be determined whether the injuries sustained will be life-changing or not, the force said.
Corrosive substances carry a minimum jail term of six months, under new sentencing guidelines for England and Wales.
The force is appealing for anyone who witnessed or has captured footage of the incident to contact them.
No arrests have been made.