Police are treating the blast as a terrorist incident after commuters reported a "fireball flying down a carriage" as the bomb, hidden in a Lidl bag, partially detonated inside the carriage.
Bomb squads and counter-terror cops are at the scene as it was revealed that the homemade bomb, likened pressure cooker devices used in 2013's Boston Marathon attack, did not fully detonate.
Theresa May is being keep informed of developments, in what appears to be the fifth major terror attack on British soil in six months.
The Prime Minister has confirmed she will chair an emergency meeting of Cobra this afternoon.
She tweeted: My thoughts are with those injured at Parsons Green and emergency services who are responding bravely to this terrorist incident."
The attack follows four previous attacks which saw five innocent people killed in Westminster in March, 22 killed in the Ariana Grande bombing in Manchester in May, eight killed in the van and knife rampage on London Bridge in June and one man killed when a van rammed into worshippers outside a North London mosque.