Photos from the scene after an explosion on a London Underground carriage.Read more
- A manhunt is under way to find the person behind Friday's Tube bombing
- Police are 'chasing down suspects' and trawling CCTV after Parsons Green attack
- Terror threat level raised to 'critical' - meaning an attack is expected 'imminently'
- The station reopened in the early hours of Saturday
- 29 people have been treated in hospital - police say 'most' had 'flash burns'
- The explosion on the District Line train is being treated as terrorism
- Police say it was caused by an 'improvised explosive device'
- Anyone with information, photos or video should call 0800 789 321
British police have arrested an 18-year old man in the Dover area in connection with the bombing on a Tube train at Parsons Green.
BBC Security Correspondent
This is the fourth time the UK national terror threat level has been raised to 'critical' since the system was made public in 2006.
The last time was in May this year, following the Manchester Arena bombing, when it was wrongly thought that the bomb-maker was still at large and could strike again.
In the case of Parsons Green it is perhaps surprising that it took the government so long - more than 12 hours - to raise the threat level to critical, under advice from the Joint Terrorism Analysis Centre, when it was obvious the perpetrator had neither died nor been caught.
Each time the level has gone to critical, it has only stayed at this highest level of alertness for three to four days - this is partly as it involves an unsustainably high tempo for the police, intelligence and security services.
The Daily Telegraph
Writing in the Daily Telegraph, Raffaello Pantucci, director of international security studies at the defence think tank Rusi, says attacks on public transport strike at the heart of cities and damage the economy.
While it is impossible to guarantee complete security, he says initiatives such as public awareness campaigns, as well as authorities staying on top of the terrorist threat, mean the UK is seeing only relatively limited impact events.
The Guardian suggests the incompetence of terrorists has spared hundreds of lives in recent years.
Before the improvised explosive device that failed to detonate fully on the tube train at Parsons Green, there have been many other bungled attempts to kill or maim people - including a world-be suicide bomber who set off his own device in a toilet.
Eliminating key individuals with high levels of expertise will help stop knowledge of destruction passing to potential attackers, the paper says.
Writing in The Times, crime and security editor Fiona Hamilton says that the Tube bombing at Parsons Green station is an important reminder of how vulnerable the transport network is.
While security has been improved since the 7 July in 2005, "a network with huge numbers of passengers can never be made 100% safe".
BBC Radio 5 live
Former chairman of the intelligence and security committee and Tory MP Dominic Grieve said he “accepted the argument” that cuts to the police have led to less community officers “picking up tips”.
He told Radio 5 Live that the government should “listen carefully” to critics of the cuts, as “anything which can increase our resilience against terrorism needs to be considered”.
However, he warned against “jumping to conclusions” that more police on the beat would have stopped such attacks from happening.
BBC Radio 4
More must be done by internet companies to tackle the spread of information online which inspires attacks, Security Minister Ben Wallace tells the Today programme.
That includes taking down manuals on how to build bombs like the one used in Parsons Green.
We take a lot down, but sometimes, very quickly, these things reappear, so we go at them again."
We have to find a way to build pressure on internet companies to get them to invest some of their vast profits into the technologies to make sure we take down content very quickly or don't even let it surface."
BBC Radio 4
The MI5 website describes what each terrorism threat level means. The last time it was raised to "critical" was following the Manchester terror attack in May.
But Security Minister Ben Wallace tells the BBC Today programme that the Parsons Green attack is different.
Unlike Manchester, where the bomber obviously killed himself along with all the innocent people he murdered, there was no obvious bomber attached to this and so there is effectively a manhunt.
We are trying to track down who did it. Whether it’s a bomber, or bombers, and all those other issues we need to answer. In this case, that’s what imminent means."
BBC Radio 4
The national terror alert level was raised to critical later on Friday evening, hours after Friday morning's bombing.
Asked why it took hours to increase the level, Security Minister Ben Wallace tells the BBC's Today programme:
The threat level in this country is set independently of politicians by the Joint Terrorist Analysis Centre.
They look at a range of things. They look at the intelligence at their fingertips. They look at changes in people we are currently watching - and see if they are inspired by an attack to maybe copy things - and they look at the pace of the investigation.
If they feel as a result of all those factors that an attack is likely to become imminent then that's why they raise it to critical as they then did later in the day."
Extra police officers are on duty on Saturday after the bombing, including from the British Transport Police.
And the police have asked asked the Prime Minister Theresa May for authorisation to enact part of the first phase of Operation Temperer - which provides military support to police force operations.
A manhunt is under way to find the person behind Friday's rush hour Tube bombing in south-west London.
Police say they are "chasing down suspects" and have hundreds of officers trawling through CCTV following the District Line attack, which injured 29.
The UK terror threat has been raised to critical - the highest level - meaning an attack may be imminent.
The station in south-west London has reopened.
Police Scotland says it will be increasing the number of armed police on patrol, particularly at key locations and crowded places.
Officers will also be reviewing all significant events taking place in Scotland over the next few days and will increase security where appropriate.
Police Scotland point out that there is no intelligence to suggest there is any specific threat to Scotland but ask the public to remain alert and report anything suspicious.
British Transport Police also say they'll have more officers - including armed police - at stations as well as on trains.
"Our hearts and prayers go out to the people of London, who suffered a vicious terrorist attack today. I spoke with a wonderful woman, British prime minister Theresa May, this morning and relayed America's deepest sympathy, as well as our absolute commitment to eradicating the terrorists from our planet. Radical Islamic terrorism, it will be eradicated, believe me."
The BBC's Daniel Sandford asks Assistant Commissioner Rowley if they know who left the device...
Assistant Commissioner Mark Rowley said he requested permission from ministers to use members of the military for armed police duty.
He said it meant he had an extra 1000 armed police available to him.
He urged members of the public to "act on their instinct" if they saw anything concerning, and "not to hesitate" to call.
Metropolitan Police Assistant Commissioner Mark Rowley says their investigation is making "excellent progress".
He told reporters "hundreds of police officers are trawling through CCTV".
Detectives have spoken to "tens" of witnesses and 77 images and videos have been sent from members of the public.
The remnants of the device has been made safe, he said, and are being examined by forensic scientists.
Home Affairs Correspondent
A word of caution about "imminence". The terror threat level was previously raised to critical in May after Manchester. Then it was lowered again days later after it became clear to intelligence assessors in the Joint Terrorism Analysis Centre that an attack wasn't imminent. Then we had two more incidents - Borough Market/London Bridge and Finsbury Park.
What does this tell us? Intelligence is usually fragmentary. Analysts sometimes only have glimpses or impressions of what they think is going on. It's an imperfect world.
Mrs May says the military will take on some of the armed police's role.
The BBC's Daniel Sandford says people "shouldn't be surprised" to see soldiers on the streets.
The terror threat level has been raised to "critical". Here's what the levels mean, according to the MI5 website:
- LOW means an attack is unlikely
- MODERATE means an attack is possible, but not likely
- SUBSTANTIAL means an attack is a strong possibility
- SEVERE means an attack is highly likely
- CRITICAL means an attack is expected imminently
Theresa May has announced that the UK's threat level has been raised to "critical".
This means "an attack is expected imminently", according to security services.
So-called Islamic State has said it was behind the Parsons Green attack, according to a statement on its Amaq news agency.
There is a greater police presence at railway stations this evening. This officer was photographed at Euston station.
The spokesperson for the UN secretary-general has been speaking about the explosion:
We are obviously following and tracking the developments in London this morning, the attack which left many people injured. We condemn the attack and we wish a speedy recovery to those who were injured. The UN stands in solidarity with the people and government of the United Kingdom and we hope the perpetrators or perpetrator will be quickly brought to justice."