The Metropolitan Police said a “mild acidic liquid” was thrown over a number of people in Ladbroke Grove, west London, during the carnival just before 8pm on Sunday.
A spokesman said: “Some of the crowd in the area then quickly dispersed, causing injuries to two people, who have since received treatment from the London Ambulance Service at the scene.
“Three people have also reported skin irritation injuries, although these are not thought to be serious.”
There have been no arrests, and police are working to identify who was responsible, the force said.
The Met said there had been 122 arrests on Sunday, a fall of about 20 per cent on last year.
The carnival, which takes place over the August Bank Holiday weekend, has seen thousands to take to the streets of west London over two days to celebrate Afro-Caribbean culture with the parade of bright colours and the bang of steel drums.
This year the festival held a minute’s silence for the 80 victims of the Grenfell Tower fire, when the sound systems were turned off.
Many of the festival’s colourful floats pass within half a mile of the blackened tower.
Register for free to continue reading
Registration is a free and easy way to support our truly independent journalism
By registering, you will also enjoy limited access to Premium articles, exclusive newsletters, commenting, and virtual events with our leading journalists
Already have an account? sign in