"Three victims - a woman and her two young children - have been taken to hospital for treatment," the police said in a statement, adding that three other people - all adults - were taken to hospital for injuries.
Three police officers were also taken to hospital with minor injuries, the statement said.
There have been no arrests at this stage, the police said.
"Firefighters were called to a chemical incident on Lessar Avenue in Clapham this evening," the London Fire Brigade said in an emailed statement to Reuters.
Crews used specialist equipment to detect a corrosive substance that is believed to have been used in the incident, it added, without giving any further details.
(Reporting by Gnaneshwar Rajan and Kanjyik Ghosh in Bengaluru; Editing by Daniel Wallis and Sandra Maler)
