Theresa May condemns 'cowardly' London Underground attack
Witnesses described scenes of terror and panic as the blast sent a "fireball" and a "wall of flame" through a District line service at Parsons Green station in west London.
Police suspect the explosion was sparked when an improvised explosive device was detonated, and have launched a huge counter-terrorism investigation alongside MI5.
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Hide AdThe device is being forensically examined while detectives are scouring CCTV and examining witness statements for potential leads.
London Mayor Sadiq Khan told LBC radio "there is a manhunt under way as we speak", while Scotland Yard said detectives are making "fast-time" inquiries to establish who was responsible, adding that there have been no arrests.
Prime Minister Theresa May condemned the "cowardly attack", saying it involved a device "clearly intended to cause significant harm".