Manhunt under way after explosion on packed London Underground train

Witnesses described scenes of terror and panic as the blast sent a “fireball” and a “wall of flame” through a District line service at Parsons Green station in west London.

Police suspect the explosion was sparked when an improvised explosive device was detonated. They have launched a huge counter-terrorism investigation alongside MI5.

The device is being forensically examined while detectives are scouring CCTV and examining witness statements for potential leads.

London Mayor Sadiq Khan told LBC radio “there is a manhunt under way as we speak”, while Scotland Yard said detectives are making “fast-time” inquiries to establish who was responsible, adding that there have been no arrests.

Prime Minister Theresa May condemned the “cowardly attack”, saying it involved a device “clearly intended to cause significant harm”.

As Britain faced up to another terrorist incident, following four attacks already this year:

:: The Prime Minister said the terror threat level would remain at severe – meaning an attack is highly likely – but would be kept under review;

:: Images emerged on social media appearing to show wires protruding from a flaming bucket inside a plastic carrier bag on the floor of a carriage;

:: Reports suggested the device had a timer;

:: A total of 22 patients were being treated in hospital, with a number of those hurt suffering from burns;

:: Scotland Yard and the PM rebuked US President Donald Trump over his claim the Parsons Green Tube bomber was “in the sights” of Scotland Yard.

Police were called at around 8.20am on Friday morning following reports of a fire on the District line train as it pulled in to the station.

Emergency services including armed police went to the scene and cordoned off the station.

Sylvain Pennec, a software developer from Southfields, near Wimbledon, was about 10 metres from the source of the explosion when fire filled the carriage.

“I heard a boom and when I looked there were flames all around,” he said.

“People started to run but we were lucky to be stopping at Parsons Green as the door started to open.”

He described the scene of panic as commuters struggled to escape the carriage, “collapsing and pushing” each other.

Mr Pennec stayed behind to take a closer look at what he believed was the source of the explosion.

“It looked like a bucket of mayonnaise,” he said.

Chris Wildish, who was on the train, said he saw a “device” in the last carriage.

He told BBC 5 live: “Flames were still coming out of it when I saw it and had a lot of wires hanging out of it – I can only assume it was done on purpose.”

Assistant Commissioner Mark Rowley, the country’s most senior counter-terrorism officer, said “many urgent inquiries” are ongoing.

He said: “We now assess this was a detonation of an improvised explosive device.”

Asked if it was clear whether the person responsible was on the train at the time, Mr Rowley said: “It’s very much a live investigation and we are following down the lines of inquiry as I said.”

Mr Rowley said Londoners can expect to see an “enhanced police presence” and called on anyone with information to contact authorities as a matter of urgency.

The blast comes with the UK on high alert following four terrorist attacks so far 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 showed terror-related arrests in Britain have hit a new record high, with suspects held at a rate of more than one every day.

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.

As of 5.30pm on Friday, NHS England said it was now treating 21 patients.

Eight others were discharged earlier in the day.