Emergency services have been called to a London Underground station following reports of an explosion on a train.
The incident occurred at Parsons Green Tube station on Friday morning.
There are reports that some passengers have 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.
May rebukes Trump
Theresa May has 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.”
The Prime Minister condemned the “cowardly” Parsons Green terror attack saying:
“This was a device intended to cause significant harm.”
She said the terror threat level would remain at severe - meaning an attack is highly likely - but would be kept under review as the investigation progresses.
Speaking in Downing Street after chairing a meeting of the Cobra emergency committee she said there would be an increased armed police presence on the transport network in London.
“The police and the security services are obviously doing all the work to discover the full circumstances of this cowardly attack.”
Manhunt underway - Cobra meeting taking place
A manhunt has been launched after a bomb was detonated in a suspected terror attack on a packed London Underground train.
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.
The device is being forensically examined while detectives are scouring CCTV and examining witness statements for potential leads.
Prime Minister Theresa May is chairing a meeting of the Government’s Cobra emergencies committee.
NHS: 22 people treated in hospital
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.
Trump slams 'sick and demented' attack
Donald Trump has condemned the Parsons Green Tube terror attack, saying those responsible were “sick and demented people”.
In a tweet, the US president said the perpetrators were “in the sights” of Scotland Yard and called for a “proactive” response.
“Another attack in London by a loser terrorist,” he wrote. “These are sick and demented people who were in the sights of Scotland Yard. Must be proactive!”
The Metropolitan Police would not be drawn on Mr Trump’s comments. “It is not something we would comment on,” a spokeswoman said.
Blast was caused by improvised explosive device
Assistant Met Police commissioner Mark Rowley says it is believed the explosion was caused by an improvised explosive device (IED).
UPDATE: 18 people injured
London Ambulance have issued an update on casualties:
How is the incident being investigated?
Details about the Parsons Green blast are still emerging, but Scotland Yard had declared a “terrorist incident” within two hours of the first emergency calls.
Investigators will be scouring CCTV and taking statements from dozens of witnesses as they attempt to piece together the lead-up to the explosion.
Pictures circulating on social media appeared to show wires protruding from a bucket inside a plastic carrier bag, but officers have so far said it is “too early” to confirm the cause of the fire.
If there is a device, it will be forensically assessed after being made safe, while police will be urgently trying to establish whether someone attempted to detonate it, and whether they were present at the scene.
First video from scene
The video shows the initial response from emergency services.
Passenger describes 'absolute chaos'
Ryan Barnett, 25, said:
“I was lucky, I wasn’t in the carriage, I was in a few carriages down but I was caught up in the stampede down the stairs.
“I was sitting there, headphones in, at Parsons Green, the doors open fine, I’m not really paying attention and all of a sudden hundreds of people run past me screaming a mixture of ‘stampede’, ‘attack’, ‘terrorist’, ‘explosion’, ‘get off the train’, ‘everyone run’.
“Parsons Green station has a staircase at the front of the carriage where the train pulls up - I’m running and keeping my head down because there might be gunfire and on to the staircase.
“But when I was on the staircase, the stewards and other passengers, they are shouting ‘stop, stop, stop’, so I ended up squashed on the staircase, people were falling over, people fainting crying, there were little kids clinging on to the back of me.
“It is absolute chaos, it was quite scary because at one stage we thought we might be trapped there - I heard a pregnant woman lost her shoes and had fallen over.”
Hospital declares 'major incident'
St Mary’s is about 4 miles from Parsons Green:
Mayor of London: 'We will never be intimidated or defeated by terrorism'
Mayor of London, Sadiq Khan, has issued a statement:
Prime Minister issues brief statement
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.”
Image: Injured woman escorted away
Scotland Yard said a number of people have been injured.
What we know so far
We’ve just published a full round-up of what we know so far about this morning’s incident at Parson’s Green.
Train can carry 865 passengers
The explosion on the London Underground occurred on an S-Stock train which can carry more than 800 passengers.
The S-Stock were the first Tube trains to have walk-through carriages to boost capacity when they were introduced to the network in 2010. Alarmed passengers would have theoretically been able to run from one end of the train to the other because of the lack of interconnecting doors.
S7 trains are used on the District, Circle and Hammersmith & City lines, and consist of seven carriages. The trains have 356 seats and a total capacity of 865 passengers.
Witnesses described the train involved in the explosion as “absolutely packed”.
The incident happened at around 8.20am, just after the busiest time of day at Parsons Green station, which is between 8am and 8.15am.
New images from scene
There’s a huge emergency services presence in the area.
Scotland Yard confirm a 'number of people' injured
Scotland Yard said “a number of people” had been injured, adding that it was “too early” to confirm the cause of the explosion which will be investigated by its Counter Terrorism Command.
Prime Minister Theresa May is to chair a meeting of the Government’s Cobra emergencies committee this afternoon.
Met Police update
Here’s what the Met Police are saying:
The Met’s Counter Terrorism Command are investigating after the incident at Parsons Green tube station is declared a terrorist incident.
Police were called at approximately 8.20am to Parsons Green station following reports of a fire on the train – emergency services remain at the scene.
It is too early to confirm cause of fire - that is now subject to the investigation that is underway.
The station remains cordoned off. Please avoid the area.
Anyone who has any info that would assist is urged to contact the hotline 0800 789 321; if it’s an emergency always call 999.
Police declare a terrorist incident
The Metropolitan Police have just declared a terrorist incident at Parsons Green tube station.
BBC: Explosion 'being treated as terrorism'
Counter-terrorism officers are reported to be at the station.
Police: 'No reports of second bomb or knifeman'
Scotland Yard has said that - contrary to earlier claims by eyewitnesses and on social media - they’ve had no reports of a second bomb or knifeman on the loose.
It was reported on social media earlier a knifeman was at large outside the station and soldiers were attempting to detonate a second device on the tube.
Terrified passengers 'sprinted along tracks'
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.”
Passenger saw 'device'
Chris Wildish, who was on the train, said he saw a “device” in the last carriage.
He told BBC 5 Live:
“It was a white bucket, a builder’s bucket, in a white Aldi bag or Lidl bag. 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.”
LIVE scene at Parsons Green
Here’s the scene LIVE at Parsons Green:
PM 'receiving regular updates'
Downing Street said Prime Minister Theresa May was “receiving regular updates” on the situation at Parsons Green.
Huge emergency service presence
A 100-metre cordon was erected around the station.
At around 9.40am five Metropolitan Police vans arrived on the scene along with a British Transport Police command van.
A police helicopter was also overhead.
'She seemed to have burns all over her body from top to toe'
BBC News presenter Sophie Raworth, 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 saw people who had burns on their hair, burns on their head and the doors opened immediately.
“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.”
Video: Crowds of people leaving Parsons Green station
This shows some of the chaotic scene in the aftermath of the incident at Parsons Green station:
Image shows burned bucket on train
This image, widely shared on social media, appears to show a burning bucket in a carriage on the train.
Six fire engines sent to scene
London Fire Brigade said six fire engines, a fire rescue unit and around 50 firefighters and specialist officers were sent to the scene.