Detectives are scouring CCTV and examining witness statements for potential leads following the latest terror attack to hit London.
An explosion at Parsons Green underground station in the west of the capital injured passengers and led to a "major incident" being declared.
Yet more people were hurt during the crush to leave the train, witnesses said.
The blast sent a "fireball" and a "wall of flame" through a packed London Underground train.
This evening, the Islamic State terror group has claimed responsibility for the attack.
The suspect has been identified on CCTV footage and a manhunt is now underway as Prime Minister Theresa May has condemned the "cowardly" attack, saying: "This was a device intended to cause significant harm."
Assistant Commissioner Mark Rowley said police were "chasing down suspects" over the attack.
The terror threat level has been raised from severe to critical, indicating a further attack may be imminent.
It is reported that commuters were left with facial burns following the incident, while others were injured in a resulting stampede after the rush hour explosion.
Scotland Yard has said there was an explosion on the train as a result of an IED device on board the tube.
Mark Rowley, Assistant Commissioner for Specialist Operations of the Metropolitan Police, says police are being supported by counter intelligence colleagues from MI5 and has reassured Londoners there will be an increased police presence in the city after the incident.
London Ambulance Service confirmed 22 people are being treated in hospital following the explosion and none are thought to be in a life-threatening or serious condition.
Mr Rowley said it was understood that "most" of the 18 injured were suffering from "flash burns".
Police are now scouring CCTV footage and speaking to witnesses.
The device used did not fully detonate, Sky News is reporting, citing unnamed sources.
Rowley declined to answer whether the authorities knew who was responsible or if the suspected bomber had been on the train, saying it was a live investigation which was being assisted by the intelligence services.
BBC has reported that the device may have been on a timer.
Prime Minister Theresa May has said that her "thoughts are with those injured and the emergency services who once again are responding swiftly and bravely to a suspected terrorist incident".
Mayor of London Sadiq Khan has appealed for calm saying London "will never be intimidated or defeated by terrorism".
The incident is the fifth terror attack to hit London this year.
Sophie Raworth, a BBC News presenter who was at the scene, said she saw a woman on a stretcher with burns to her face and legs.
"I spoke to a woman, Laura Bishop, who said she was standing there and all of a sudden heard a bang - and saw a wall of flames, flames in the carriage," she said.
"She saw people who had burns on their hair, burns on their head and the doors opened immediately."
Ms Raworth added: "I have just seen a woman who was just stretchered off here and clearly her legs are wrapped up and she has burns.
"She had burns to her face - she's conscious, she was taking oxygen and pain relief as well.
"She seemed to have burns all over her body from top to toe."
Speaking on the Victoria Derbyshire show, she told how a witness told her there was a stampede and then "panic, just complete panic".
She added: "There are a lot of people here at Parsons Green looking very anxious and worried, trying to work out what has happened. Clearly just trying to get out of the station itself was pandemonium."
Foreign Affairs Minister Simon Coveney has said there are no indications of any Irish involvement,
“I am shocked by the incident in London this morning, which the Metropolitan Police are treating as a suspected terrorist attack. My Department is monitoring the situation closely through the Irish Embassy in London and the Consular Division in Dublin. As of now, we have no indications to suggest that any Irish citizens have been affected," he said in a statement.
"Anyone with concerns for loved one in London can contact my Department on 00353 (1) 4082527. On behalf of the Irish Government, I wish to express our solidarity with the people of London at this time.”
Transport for London has said there is no service between Edgware Road and Wimbledon on the district line.
A picture posted social media appeared to show a white bucket in a bag that "exploded".
A Twitter user said: "Fireball flew down carriage and we just humped out open door."
Explosion on Parsons Green district line train. Fireball flew down carriage and we just jumped out open door. pic.twitter.com/pGbfotbfsJ— Rigs (@el_rigs) September 15, 2017
A video has also emerged showing the item smoldering in the carriage.
Reacting to the attack, Taoiseach Leo Varadkar said: “We’ve been in touch with our embassy in London in relation to the suspected terrorist attack that occurred on the London Underground today.
“As of yet there are no Irish citizens understood to have been involved. We will continue to keep in touch with the authorities in London and a helpline has been established for anybody is seeking information.”
US President Donald Trump has denounced the incident as an "attack by a loser terrorist".
"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!," he wrote on Twitter.
"Loser terrorists must be dealt with in a much tougher manner.The internet is their main recruitment tool which we must cut off & use better!
"The travel ban into the United States should be far larger, tougher and more specific-but stupidly, that would not be politically correct!"
Ms May has rebuked the president over his comments saying: "I never think it's helpful for anybody to speculate on what is an ongoing investigation."
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!— Donald J. Trump (@realDonaldTrump) September 15, 2017
A woman at London's Parsons Green underground train station said she was injured in a stampede.
Outside the station, a woman was sitting on a pavement with a bandage around her leg while armed police patrolled.
BBC correspondent Riz Lateef, who was at the station travelling to work, said there was "panic as people rushed from the train, hearing what appeared to be an explosion.
"People were left with cuts and grazes from trying to flee the scene. There was lots of panic," she said.
Media technology consultant Richard Aylmer-Hall, 53, was sitting on the District Line train bound for central London when panic unfolded at about 8.20am. Several people were injured after passengers tried to escape, he said.
He added: "Suddenly 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.
"Some people got pushed over and trampled on, I saw two women being treated by ambulance crews."
"I was on second carriage from the back. I just heard a kind of whoosh. I looked up and saw the whole carriage engulfed in flames making its way towards me," a man who was on the train told Reuters.
Sylvain Pennec, a software developer from Southfields, near Wimbledon, was around 10 metres from the source of the explosion when fire filled the carriage.
"I heard a boom and when I looked there were flames all around," he said.
"People started to run but we were lucky to be stopping at Parsons Green as the door started to open."
He described the scene of panic as commuters struggled to escape the carriage, "collapsing and pushing" each other.
Mr Pennec stayed behind to take a closer look at what he believed was the source of the explosion.
"It looked like a bucket of mayonnaise," he said.
"I'm not sure if it was a chemical reaction or something else, but it looked home-made. I'm not an expert though."
Chris Wildish, who was on the train, said he saw a "device" in the last carriage.
"It was a white bucket, a builder's bucket," he told BBC 5 live.
"Flames were still coming out of it when I saw it and had a lot of wires hanging out of it - I can only assume it was done on purpose.
"It was standing against the door of the rear-most carriage."
A woman has described how she kept running with the crowd, despite not knowing what had happened on the train.
Teacher Sally Faulding told BBC News her colleague told her she heard a "whoosh" sound before seeing flames coming towards her.
"I was in the middle of the train.
"Suddenly I saw people coming towards me.
"I didn't know what was going on but I knew something was wrong because of the volume of people coming towards me.
"The doors opened and I just ran out as everyone else did.
"We were still running on the platform and people were falling down, trying to make an exit.
"I fell over someone who fell in front of me," Ms Faulding continued.
"I kept on running even though I still didn't know what I was running from.
"Basically we moved out of the station. People were crying, visibly upset.
"I then heard people saying that there had been an explosion, a fire."
Ms Faulding said she bumped into a colleague of hers who confirmed she had seen a fire.
"She said she saw a surge of fire coming toward her.
"She heard a whoosh sound before that.
"I saw a man who had burns on his arms and a woman who had her hair singed."
Police are urging Londoners to avoid the area.
They have urged people to contact them if they have information which could assist with their investigation:
"Anyone who has information that would assist detectives is urged to contact the hotline 0800 789 321; if it's an emergency always call 999.
"Anyone with images from the scene is asked to upload them to www.ukpoliceimageappeal.co.uk."
Additional reporting by agencies