The explosion devastated the District Line Tube train at Parsons Green station, in west London.
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” during the Parsons Green terror attack.
Prime Minister Theresa May has confirmed she will chair a meeting of the Government’s Cobra emergencies committee this afternoon at Downing Street.
The attacker is now feared to be at large.
The Met Police said “several people” had been injured in the London terror attack. St Mary’s Hospital has declared a “major incident”.
One woman said on Twitter: “I’m safe – just had to run for my life at #ParsonsGreen station – huge stampede, lots injured. Not sure why – fire/explosion mentioned.”
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.
Local MP Greg Hands said: “[If] the perpetrator is still at large… capturing the perpetrator and attending to those injured must be the priority.”