"We are aware of an incident at Parsons Green tube station. Officers are in attendance," London's Metropolitan Police said on Twitter.
The station was closed, as well as an entire section of the District Line where it is located.
A Metro.co.uk reporter at the scene was quoted by the paper as saying that a white container exploded on the train and passengers had suffered facial burns.
She said they were "really badly burned" and their "hair was coming off".
London Fire Brigade also said they were at the scene and were called at 8:21 am (0721 GMT).