Latest as of 2100hrs on Friday 15 September:
Counter terrorism detectives are urging anyone with information about the terrorist attack at Parsons Green this morning to contact police.
Twenty-nine people are known to have been injured during the attack in which an improvised explosive device was detonated on a tube train at Parsons Green Tube Station at around 08:20hrs this morning.
Assistant Commission Mark Rowley, head of National Counter Terrorism Policing, said: “Our investigation is continuing at speed and our priority is to identify, locate and arrest those responsible.
“Hundreds of police officers are pursuing numerous lines of enquiry, trawling through hours of CCTV footage and speaking to witnesses.
“The device and remnants left at the scene have been taken away and are being examined by forensic experts. While this work is ongoing, there is no doubt in my mind that those responsible intended to cause great harm and injury.”
So far, detectives have spoken to 45 witnesses and continue to receive information from the public to the confidential anti-terrorist hotline.
The public has sent 77 images and videos to investigators via the UK Police Image Appeal website.
AC Rowley, continued: “My thoughts are with all those who have been affected by this terrible attack. It is an attack on all of us.
“I urge communities across the UK to be vigilant and report any suspicious behaviour to the police immediately. Please do not hesitate, no matter how insignificant you may think the information you have is – please let us decide. It could be crucial.”
Anyone with information is urged to call the Anti-Terrorist Hotline on 0800 789 321 or, in an emergency, always call 999.
Anyone with footage or images from the incident is urged to upload them at www.ukpoliceimageappeal.co.uk where they will be looked at by investigators.
The independent Joint Terrorism Analysis Centre has raised the threat level from international terrorism to critical. This means that their assessment is not only that a terrorist attack is highly likely but that a further attack may be imminent.
Military personnel have been drafted in to protect national infrastructure sites, allowing additional armed police officers to carry out patrols.
Communities across the UK can expect to see more officers, both armed and unarmed, on patrol by foot and in vehicles over the weekend. In particular, they will be patrolling at crowded places, iconic sites, transport hubs and ports.
This may mean disruption for those who are travelling. The public are asked to be patient and plan ahead, allowing time for delays.
Police are reviewing the security of events across the UK and working with businesses and organisers to make events as secure as possible. Security checks at events will be enhanced, so people attending events should arrive early and allow extra time for these checks. If you don’t need to take a bag, don’t take it.
Latest as of 1500hrs on Friday 15 September:
Detectives from the Met Police's Counter Terrorism Command are carrying out an urgent investigation into an incident on a London tube train this morning.
Police were called at around 08:20hrs to Parsons Green Underground Station following reports of a fire on the train travelling into London on the District Line.
Officers from the Met and British Transport Police attended the scene, along with colleagues from London Fire Brigade and London Ambulance Service.
London Ambulance Service has taken 18 people to hospital, believed to be suffering mainly from burns.
It has been declared a terrorist incident. Detectives from the Counter Terrorism Command, part of the national counter-terrorism police network, are making fast-time inquiries to establish who was responsible and are working closely with the security services. So far there have been no arrests.
We believe the fire was caused by the detonation of an improvised device, which is being forensically examined.
Assistant Commissioner Mark Rowley, head of national counter-terrorism policing, said: "There are many urgent inquiries ongoing with hundreds of detectives involved, looking at CCTV, carrying out forensic work and speaking to witnesses.
"I am appealing for anyone who has information that would assist detectives to contact the hotline on 0800 789 321. I would also urge anyone with images or moving footage from the scene to upload them at www.ukpoliceimageappeal.co.uk
"Today and over the weekend the public can expect to see a heightened police presence, particularly in crowded places and at transport hubs.
"As always we urge the public to remain calm but alert; if they have any concerns, see or hear anything suspicious then please contact the police on the confidential hotline 0800 789 321. In an emergency always call 999."
The station remains cordoned off and people are being advised to avoid the area.
Latest as of 10:10hrs on Friday 15 September:
Officers from the Met's Counter Terrorism Command are investigating after an incident on a London tube train this morning.
Police were called at approximately 08:20hrs on Friday, 15 September to Parsons Green Underground Station following reports of a fire on the train.
Deputy Assistant Commissioner Neil Basu, the senior national coordinator for CT policing, has declared it a terrorist incident.
Officers from the Metropolitan Police Service and British Transport Police attended the scene, along with colleagues from London Fire Brigade and London Ambulance Service.
At present we are aware of a number of people who have suffered injuries.
It is too early to confirm the cause of the fire, which will be subject to the investigation that is now underway by the Met's Counter Terrorism Command.
The station remains cordoned off and we are advising people to avoid the area.
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
We urge the public to remain calm but alert and if you have any concerns, see or hear anything suspicious then contact police.