Photos from the scene after an explosion on a London Underground carriage.

What does the device used on the District Line tell us about the attacker?

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.

Frank Gardner
BBC Security Correspondent

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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.

Read the full analysis here.

The Daily Telegraph

Writing in the Daily Telegraph, external, 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

The Guardian suggests, external 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.

The Times

Writing in The Times,, external 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

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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.

Today Programme
BBC Radio 4

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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.

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NHS England is offering advice on how to cope with reactions in the wake of traumatic incidents.

More details can be found here, external. Or you can go to the NHS Choices website at www.NHS.uk

Today Programme
BBC Radio 4

The MI5 website describes what each terrorism threat level means, external. 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.

Today Programme
BBC Radio 4

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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:

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.

Image source, PA

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.

Read the full story here.

The hunt for the person responsible for the attack on a train in south-west London dominates the front pages.

The station in south-west London has reopened.

Another attack may be imminent after an explosion on a Tube train in south-west London injured 29.

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.