Scotland Yard has confirmed the detonation of an improvised explosive device was responsible for the explosion on the district line tube train at Parsons Green station in South West London at around 8.20 this morning.

Assistant Commissioner Mark Rowley, addressing journalists on the steps of Scotland Yard, said that 18 people had been taken to hospital, with many of the injuries appearing to be flash burns. He said that hundreds of police officers were now involved in the live investigation led by the Metropolitan Police’s Counter-Terrorism Command and that the public could expect to see an enhanced police presence, particularly across London Transport.

The source of the explosion appeared to be contained in a white container within a supermarket carrier bag on the packed commuter tube train headed into central London. While witnesses spoke of people being trampled under foot in the panic to escape from the station, which can be exited by just one narrow stairwell, the ambulance service is yet to confirm if some of the injuries were a result of this stampede. 18 people have been taken to hospital, though none are thought to be in a serious or life threatening situation.

It is the fifth terror attack in Britain since March, including the attack on Westminster Bridge and the Palace of Westminster, the attack on the Manchester Arena, the attack on London Bridge, and the attack outside the Finbury Park Mosque in London. The current threat level for Britain is severe, meaning an attack is highly likely. The attack took place at rushhour on a train at Parsons Green station on the district line in South West London. The attack has come 12 years after multiple attacks took place on London transport: buses and tube trains, killing 52 people.

Speaking about the incident a Metropolitan Police spokesperson said they were investigating a “critical and extremely serious incident,” relating to what seemed like an “item exploding to a small degree.” He said the state of the injuries as a result of the incident were still being assessed, though surrounding areas had been closed. “At this stage we are still in the very early stages and its too early to speculate on any next steps.”

One passenger who was on the carriage where the incident took place told the BBC he had heard a large bang and then a fireball above his head, which singed his hair. “A lot of people were worse off than me,” he said. Others spoke of panic as people attempted to leave the station.

The Metropolitan Police urged the public to remain calm and alert and for those who had images of the scene to upload them to a police website. The Prime Minister Theresa May is to host a meeting of the emergency COBRA committee. “My thoughts are with those injured at Parsons Green and emergency services who are responding bravely to this terrorist incident.”

“Our city utterly condemns the hideous individuals who attempt to use terror to harm us and destroy our way of life,” said London Mayor Sadiq Khan. “As London has proven again and again, we will never be intimidated or defeated by terrorism,” he added, calling on Londoners to remain calm and vigilant.

U.S. President Donald Trump waded into controversy for yet another time following a terrorist attack in Britain as he appeared to suggest the person suspected of carrying out the terrorist attack on the tube train was known to police, even before British police had confirmed this was the case - and that the Internet should be “cut off.”

“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!,” he tweeted, adding that “loser terrorists” ought to be dealt with in a much “tougher manner. The internet is their main recruitment tool which we must cut off & use better!’ The Metropolitan Police’s Counter terrorism command who are leading the investigation have made clear that it remains a live case in its early stages, and that hundreds of officers were wading through information. They are yet to confirm if suspects were known to them or security services.

Trump triggered anger in Britain after he criticized London mayor Sadiq Khan following the June attack on London Bridge for urging people to remain calm. There was also controversy following the attack on the Arianna Grande concert in Manchester when sensitive details were released to the U.S. media, triggering concerns about the flow of sensitive information between the two countries, which work closely together on security and counter terror issues.

Trump’s latest comments provoked frank criticism including from within the British political establishment. “True or not - and I’m sure he doesn’t know - this is so unhelpful from leader of our ally and intelligence partner,” tweeted Nick Timothy, a former close advisor of Prime Minister Theresa May who resigned following the June election.