IED event in London, Parsons Green area, United Kingdom on Fri 15th September 2017

An "improvised explosive device" was detonated on a Tube train in south-west London during Friday's morning rush hour, injuring 29 people.

The blast, at Parsons Green station on a District Line train from Wimbledon, is being treated as terrorism.

So-called Islamic State says it carried out the attack, which Prime Minister Theresa May condemned as "cowardly".

A hunt is under way for the person who placed the device and the area around the station has been evacuated.

Specialist officers there securing the remains of the improvised device and ensuring it is stable.

Mrs May said the attack had been "intended to cause significant harm".

She said the UK's terror threat level would remain at severe - the second highest - but would be under review.

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.

Hundreds of detectives and MI5 are investigating the attack, which took place at 08:20 BST on an eastbound train.

Assistant Commissioner Mark Rowley earlier refused to say whether anyone had been arrested.

Pictures taken of the train show a white bucket on fire inside a supermarket bag, with wires trailing on to the carriage floor. The BBC understands the device had a timer.

BBC security correspondent Frank Gardner said the bomb appeared not to have gone off.

Had it worked as intended, it would have killed everyone around it and maimed everyone in the train carriage for life, he said.

US President Donald Trump has spoken to Prime Minister Theresa May to convey his sympathies for those injured in the terrorist attack, the White House has said.

President Trump pledged to continue close collaboration with the UK to stop attacks and combat extremism, the statement added.

A Downing Street spokesman said: "President Trump called the prime minister earlier today to offer his condolences over this morning's cowardly attack in London."

US President Donald Trump had earlier tweeted that the "sick and demented" people behind the attack had been in the sights of the Metropolitan Police, prompting Mrs May to say it was not helpful to "speculate" on an ongoing investigation.

Chelsea and Westminster Hospital has been treating 14 people, with a "small number" of them being taken to its specialist burns unit.

Four people are being treated by Imperial College Healthcare and three at St George's Hospital.

St Thomas' Hospital in London said it had treated eight patients but they had now been discharged.

Mr Rowley asked the public to remain "vigilant", but said they should "not be alarmed".

He said anyone who took pictures or videos at the scene could upload them to ukpoliceimageappeal.co.uk.

London Mayor Sadiq Khan appealed for calm, saying the city would "never be intimidated or defeated by terrorism".