Metropolitan Police spokesperson said they were investigating the “critical and extremely serious incident.”

Friday’s blast at London Underground’s Parsons Greens station is the fifth terror attack in Britain since March. The previous four were 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 Finsbury Park Mosque in London.

The current threat level for Britain is severe meaning an attack is highly likely. The Parsons Green attack has also come twelve years after multiple attacks took place on London transport: buses and tube trains, killing 52 people.

Speaking at 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.