Police have confirmed they are treating the explosion as a terrorist incident.
Eye-witnesses said victims suffered facial injuries after a what appeared to be a homemade bomb in a Lidl exploded.
One witness said there were “lots” of people injured after the explosion and a subsequent stampede.
Another said a “fireball flew down the carriage”.
A photo shared on social media appears to show the object which exploded on the train this morning.
It appears to be a white, bucket contained in a Lidl bag.
The photo showed the container still burning with passengers’ bags scattered across the carriage.
Another photo appears to show wires hanging out of the container.
He said: “I was blissfully reading my paper and listening to a podcast and suddenly the whole world charged past me down the platform, down the Tube.
“There was panic, lots of people shouting, screaming, lots of screaming.
“There was a woman on the platform who said she had seen a bag, a flash and a bang, so obviously something had gone off.
“It was an absolutely packed, rush-hour District Line train from Wimbledon to Edgware Road.
Parsons Green attack: A woman is treated following the Tube train explosion
“Some people got pushed over and trampled on, I saw two women being treated by ambulance crews.
“Since then, every emergency vehicle in London has gone past me, fire engines, ambulances, every type of police vehicle.
“I don’t think anyone was hurt by the actual device or whatever it was.”