London attack – as it happened: Theresa May raises threat level to critical as Isis claims responsibility

Here's a report from the Press Association's Hayden Smith on how the incident will be 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.

The scenes of panic evoked memories of the July 7 atrocities in 2005, when suicide bombers killed 52 people in a series of co-ordinated attacks across the transport network.

Two weeks later, a group of men attempted to set off rucksack devices but the bombs failed to go off.

The emergency at Parsons Green will spark another huge counter-terrorism probe as security services confront an unprecedented threat.

Four attacks - at Westminster, Manchester, London Bridge and Finsbury Park - have hit Britain already this year.

Authorities have foiled 19 plots since the middle of 2013 - including six since the Westminster atrocity in March.

The incident comes a day after figures revealed terror-related arrests in Great Britain have hit a new record high, with suspects held at a rate of more than one every day.

Police warned there is no such thing as a "typical terrorist" after official statistics showed rises in numbers detained across ethnicities and age groups.

There were 379 arrests for terrorism-related offences in the year ending June 2017, the highest number in a 12-month period since data collection began in 2001.

Police and MI5 are running 500 investigations involving 3,000 individuals at any one time, while there are also 20,000 former "subjects of interest" whose risk must be kept under review.

18 people have been injured and gone to hospital, according to reports. None are thought to be in a serious or life-threatening condition, the London Ambulance Service said.

It's not clear how man of those were hurt by the explosion, and how many by the stampede that followed.

Nigel Holness, London Underground's director of network operations, said: "Our staff have worked to assist the emergency services and to ensure the safety of our customers during this morning's incident at Parsons Green.

"We are focussed on assisting the police investigation into what happened and providing support for our customers and staff.

"Our Sarah Hope Line is available to offer help and support to anyone affected by this morning's incident."

Jeremy Corbyn has tweeted: "Thoughts with those injured in Parsons Green terrorist incident, and thanks to police, ambulance staff and firefighters who are responding."

Scotland Yard confirms that "that this was a detonation of an improvised explosive device".

The police asks that anyone with information call them as a matter of urgency.

Donald Trump has responded to the attack: "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!"

Donald Trump added: "Loser terrorists must be dealt with in a much tougher manner.The internet is their main recruitment tool which we must cut off & use better!"

Donald Trump continued: "The travel ban into the United States should be far larger, tougher and more specific-but stupidly, that would not be politically correct!"

The BBC reports the improvised explosive device used at Parsons Green had a timer.