A huge counter-terrorism investigation has been launched after a bomb was detonated on a packed London Underground train.
Witnesses described scenes of terror and panic after the blast sent a "fireball" and a "wall of flame" through a District line service at Parsons Green in West London.
Britain's top counter-terror officer Mark Rowley said: "We now assess this was a detonation of an improvised explosive device."
Eighteen people were injured in the explosion, with most of them suffering "flash burns".
US President Donald Trump denounced the attack on Twitter saying: "Another attack in London by a loser terrorist. These are sick and demented people who were in the sights of Scotland Yard. Must be proactive!"
The Met declined to comment on Mr Trump's claim.
Mr Rowley said the investigation is being led by the Met's counter-terrorism command, with support from MI5.
Speaking outside Scotland Yard, the assistant commissioner said: "There are many urgent enquiries ongoing now with hundreds of officers involved - looking at CCTV, forensic work and speaking to witnesses.
Asked if it was clear whether the person responsible was on the train at the time, Mr Rowley said: "It's very much a live investigation and we are following down the lines of enquiry as I said."
He said Londoners can expect to see an "enhanced police presence", particularly across the transport system across the day.
And he called on anyone with information to contact authorities as a matter of urgency.
He added: "Finally I would ask the public to remain vigilant across London and to be alert in assisting us but not alarmed."
NHS England said 22 patients in total were being treated at Imperial, Chelsea and Westminster, Guy's and St Thomas' and the Central London Community Healthcare Urgent Care Centre.
Of these 18 were taken by ambulance and four self-presented.
Stephen Cowan, the leader of Hammersmith and Fulham Council, the area in which the attack took place, told the Press Association he was "outraged" by the explosion.
He said: "Around 8.20 this morning a package exploded in the final carriage of a District Line train at Parsons Green station, it is being treated as a terrorist incident.
"What is extremely impressive was that the emergency services were on to the situation immediately, there is a completely co-ordinated effort and there is no reason to be overly concerned.
"I would stress that people should be reassured because this is being dealt with very effectively by everyone in the emergency services.
"I can't tell you how angry I am, I am absolutely outraged that someone could do this - attacking innocent lives, putting those people at risk and causing fear and harm is not a way to express yourself.
"This kind of terrorism needs to be stamped out, but people need to recognise that in London we have one of the most amazing cities anywhere, they are not going to damage us, they are not going to beat us."
The Transport Salaried Staffs Association called on London's mayor to hold a safety summit following the incident.
General secretary Manuel Cortes said: "Our union has long been concerned that we are not doing all we can to maximise safety on our Tube and on London's public transport network."
Home Secretary Amber Rudd said: "Once more people going about their everyday lives have been targeted in a callous and indiscriminate way.
"My thoughts are with all those injured and affected.
"The police and emergency services were quickly on the scene and I would like to pay tribute again to their professionalism and tireless commitment.
"I am receiving regular updates on what is an ongoing and active investigation.
"I would urge Londoners to remain alert and assist the police and emergency services as much as they can."
Mayor of London Sadiq Khan told LBC: "There is a manhunt under way as we speak."