Emergency services gather close to the 'Parsons Green' Underground Station in London, Britain, 15 September 2017. Emergency services have responded to reports of an explosion on an underground tube train.Emergency Services respond to incident at Parsons Green Underground Station, London, United Kingdom - 15 Sep 2017

LONDON — An explosive device went off on Friday morning at a London Underground station in West London, the Metropolitan Police Service has confirmed. The incident, in which 30 people were injured, is being treated as a terrorist attack.

An 18-year-old man was arrested at the port of Dover on Saturday morning, and police are continuing to search a house in Sunbury-on-Thames in Surrey. It is thought the 18-year-old lives there with an elderly couple who were rewarded for their service to children when they were made MBEs in 2010. Neighbors said that the teen had been in trouble with the police before.

Police said a 21-year-old man was arrested in Hounslow, west London, on Saturday night on suspicion of a terror offense and is in custody in south London.

Prime Minister Theresa May has raised the UK terror threat level from “severe” to “critical,” meaning an attack is considered to be “imminent.”

A British Fashion Council spokesperson said, “We follow strict security protocols in line with all major events and take the security of  London Fashion Week and safety of those visiting and participating incredibly seriously; ensuring that the event will run smoothly.”

Police were called at approximately 8.20 a.m. on Friday, the first day of London Fashion Week, following reports of a fire on the train. Images show the explosive device to be a white bucket with wires emerging from it.

It exploded as the train was pulling into the station during rush hour. Some reports say that a carriage of the train was engulfed in flames, others describe a puff of smoke and a chemical smell.

Passengers were evacuated from the rear of the train onto the tracks at the aboveground station, with witnesses describing a panicked “stampede” down the stairs to exit the station.

Dozens of ambulances, fire trucks and police vehicles rushed to the scene. Uniformed and plain-clothes police officers, members of the counterterrorism command, the British Transport Police, and London Ambulance Service’s Hazardous Area Response Team have been deployed at the station and in surrounding streets.

“It now seems there was a detonation of an improvised explosive device,” said Mark Rowley, assistant commissioner at the Metropolitan Police Service, on Friday morning. He said there would be an increased police presence, particularly on transport, during the rest of the day. Police are “keeping an open mind” on the number of suspects involved.

Twenty-two people were taken to hospital, according to Natasha Wills, assistant director of operations at London Ambulance Service. None are thought to be in a serious or life-threatening condition, with most believed to be suffering from burns and injuries caused in the stampede from the station. Of the 30 people injured, one is still being treated in hospital.

There are several schools in the vicinity of the station, and the attack coincided with the time when many children were on their way to school. Residents of the streets surrounding the station were been evacuated to the White Horse pub on Parson’s Green.

Inside, one woman told WWD that she had seen a young boy with grievous injuries to his feet and that her neighbor in one of the apartment blocks at Dairy Close, which overlooks the station, had taken photographs of three men fleeing the station immediately after the attack, running along rooftops to make their escape. The eyewitness was unavailable for comment as police said she had been escorted from the pub for questioning.

Another witness noted that images she had taken of the train had mysteriously vanished from her phone and were no longer accessible to view.