London Police are reviewing CCTV in the hunt for those responsible for the terrorist blast on a tube train this morning.
It is understood the footage shows a suspect placing the device on the train.
Twenty-two people were injured at Parsons Green station when the homemade bomb partially detonated.
The Taoiseach has said the Irish Embassy in London has reported to the government that no citizens are caught up in today's events.
Foreign Affairs Minister Simon Coveney said he is "shocked by the incident".
Major General Chip Chapman, the former Head of Counter Terrorism at the Ministry of Defence in the UK, said the response will already be well advanced.
"The security services and the police will already be asking is this a lone wolf? Which I suspect it's not. It part of a network? Is there a further attack to come?
"Of course, the Joint Terrorism Analysis Centre will be looking at do we need to do anything about the threat level if they think there's a network," he added.
Scotland Yard said the area surrounding Parsons Green Tube station has been evacuated so specialist officers could "secure the remnants of the improvised device and ensure it is stable".
A spokesman added: "A small number of residents will be affected by the evacuation, which encompasses an area approximately 50 metres in radius."
The Tube station remains closed.
The evacuation has been necessary whilst officers secure the remnants of the improvised device & ensure it is stable #ParsonsGreen— Metropolitan Police (@metpoliceuk) September 15, 2017
The UK Prime Minister Theresa May has condemned the "cowardly" Parsons Green terror attack, saying: "This was a device intended to cause significant harm."
She also delivered a rebuke to Donald Trump over his claim the Parsons Green Tube bomber was "in the sights" of Scotland Yard.
Following a meeting of the Government's Cobra emergencies committee, the Prime Minister said: "I never think it's helpful for anybody to speculate on what is an ongoing investigation."
London Ambulance said it had taken 19 patients, mostly with minor injuries, to three London hospitals - St Mary's, St Thomas' and the Chelsea and Westminster.
PM: My thoughts are with those injured at Parsons Green and emergency services who are responding bravely to this terrorist incident.— UK Prime Minister (@10DowningStreet) September 15, 2017
Imperial College Healthcare NHS Trust, which earlier declared a major incident at St Mary's Hospital to deal with the attack, tweeted: "St Mary's Hospital has now been stood down from the major incident."
Mayor of London Sadiq Khan told LBC: "There is a manhunt under way as we speak."
A huge counter-terrorism investigation has been launched after a bomb was detonated on a packed London Underground train.
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."
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."
18 people were taken to hospital after the Parsons Green explosion, London Ambulance Service said.
Mr Rowley said it was understood that "most" of the 18 injured were suffering from "flash burns".
An explosion on a packed Tube train in Parsons Green, west London, is being treated as a terrorist incident, Scotland Yard said.
Scotland Yard has declared a terrorist incident after a blast sent a "fireball" and a "wall of flame" through a packed London Underground train.
A major incident has been declared at St Mary's Hospital in Paddington in response to the Parsons Green explosion.
Prime Minister Theresa May is to chair a meeting of the Government's Cobra emergencies committee this afternoon to discuss the terrorist incident at Parsons Green.
The PM said: "My thoughts are with those injured at Parsons Green and the emergency services who, once again, are responding swiftly and bravely to a suspected terrorist incident."
Pictures posted on social media appeared to show wires protruding from a flaming bucket inside a plastic carrier bag on the floor of a carriage.
The Metropolitan Police urged anyone with photographs from the scene to upload them to www.ukpoliceimageappeal.co.uk to aid the investigation.
Emergency services were called to a London Underground station after reports of an explosion on a train.
Pictures appeared to show an explosion in a bucket towards the rear of a train.
The incident occurred at Parsons Green Tube station on Friday morning.
A metro.co.uk reporter said passengers suffered facial burns.
The Metropolitan Police said: "We are aware of an incident at Parsons Green Tube station. Officers are in attendance."
British Transport Police said: "We're aware of an incident at Parsons Green station. Officers are at the scene. The station is closed."
Transport for London said it is "investigating an incident at Parsons Green" and urged customers travelling between Wimbledon and Earls Court to use alternative routes.
A London Ambulance Service spokesman said: "We are on scene alongside the Metropolitan Police at Parsons Green."
London Fire Brigade said they were called to the incident at 8.21am on Friday.
Media technology consultant Richard Aylmer-Hall, 53, was sitting on the District Line train bound for central London when panic unfolded at around 8.20am.
He said he saw several people injured, having apparently been trampled as they tried to escape.
He told the Press Association: "I was blissfully reading my paper and listening to a podcast and suddenly the whole world charged past me down the platform, down the Tube.
"I was on the Tube, we had just stopped at Parsons Green, I was on my way up from Wimbledon towards Paddington and 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.
"It was an absolutely packed, rush-hour District Line train from Wimbledon to Edgware Road.
"I saw crying women, there was lots of shouting and screaming, there was a bit of a crush on the stairs going down to the streets.
"Some people got pushed over and trampled on, I saw two women being treated by ambulance crews.
"Since then, every emergency vehicle in London has gone past me, fire engines, ambulances, every type of police vehicle."
Mr Aylmer-Hall added: "I don't think anyone was hurt by the actual device or whatever it was."
Natasha Wills, of London Ambulance Service said: "We were called at 8.20am to reports of an incident at Parsons Green underground station.
"We have sent multiple resources to the scene including single responders in cars, ambulance crews, incident response officers and our hazardous area response team, with the first of our medics arriving in under five minutes.
"Our initial priority is to assess the level and nature of injuries. More information will follow when we have it."
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."
One passenger, named only as Lucas, told BBC 5 live: "I heard a really loud explosion - when I looked back there appeared to be a bag but I don't know if it's associated with it.
"I saw people with minor injuries, burnings to the face, arms, legs, multiple casualties in that way.
"People were helping each other."
Transport for London said Tube services were suspended between Edgware Road and Wimbledon.
In a statement, Scotland Yard said: "Police were called at approximately 8.20am to Parsons Green Underground Station following reports of an incident on a Tube train.
"Officers from the Metropolitan Police Service and British Transport Police are in attendance along with the London Fire Brigade and London Ambulance Service.
"We would advise people to avoid the area."
Another witness, Sham, said he saw a man with blood all over his face.
"There were loads of people crying and shaking," he told 5 live.
"There were a lot of people limping and covered in blood.
"One guy I saw, his face was covered in blood - I've never seen anything like it."
Mick Cash, general secretary of the Rail, Maritime and Transport union, said: "The incident shows once again that Tube staff are the first responders in emergency situations and the clear need for safety and security to remain the number one priority and for the resources and staffing to be in place to deal with all eventualities."
Downing Street said Prime Minister Theresa May was "receiving regular updates" on the situation at Parsons Green.
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, in a white Aldi bag or Lidl bag," he told 5live.
"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."
Passengers on the train behind the affected Tube witnessed terrified passengers sprinting away up the tracks.
They were then held in their train for over an hour before being evacuated on to the tracks themselves.
Nicole Linnell, 29, who works for a fashion label, said: "We saw people running down the tracks. About 30 or 40 people.
"They were running down the tracks outside our train.
"It was absolutely terrifying. Running on the tracks is the last thing you want to do so we were like 'What's going on?'
"After about an hour we were evacuated off the train on to the tracks. About 10 to 15 people at a time."
Melanie Heyside had been at a gym nearby and intended to get on a District Line train to go to work.
She told Sky News: "I just was about to open the doors to leave the studio and then all of a sudden Swat cars completely stormed and blocked the street and police jumped out with their armour and were telling people to 'move, to move'.
"So I was inside with a group of other people and we weren't really sure what to do."
She said they were told to leave the area, which she added was populated by schoolchildren and others going to work, as quickly as possible.