Detectives investigating the terrorist attack at Parsons Green have made what officials are calling a “significant” arrest in connection with the bombing.

The 18-year-old male suspect was arrested by Kent Police in the port area of Dover on Saturday morning under section 41 of the Terrorism Act.

He remains in custody at a local police station from where he will be transferred to a south London police station.

British Prime Minister Theresa May has lifted the national security threat level to critical from severe, meaning an attack is expected imminently, after an improvised bomb exploded on a London commuter train on Friday injuring 29 people.

Detectives investigating the terrorist attack at Parsons Green have made a “significant” arrest in connection with the attempted bombing.

A massive manhunt is now underway as Scotland Yard confirms they are “chasing down suspects” with hundreds of armed police and military on the streets over the next several days.

The improvised bucket bomb was planted on the District Line service at Parsons Green station in west London and exploded inside a packed train carriage during peak hour at 8.30am (local time), causing a massive stampede.

The line between Earl’s Court and Wimbledon was immediately shut down as firemen, anti-terrorism police and the bomb squad flooded the station and surrounding streets.

In a televised statement, Ms May said the move to critical was “a proportionate and sensible step”.

“For this period, military personnel will replace police officers on guard duties at certain protected sites that are not accessible to the public,” she said.

Theresa May says UK terror threat level raised to critical, highest possible level, after London Tube blast https://t.co/HCBPJZo8XY pic.twitter.com/liMAODY2IV

— BBC Breaking News (@BBCBreaking) September 15, 2017

Up to 18 people were taken to hospital with “flash” burns and several injured as panicked commuters rushed to street level. Others in the rear of the train walked out quietly in single file across the railway tracks.

One eyewitness caught up in the stampede heard one woman yelling out that she was pregnant, and saw another young boy’s face smash into a brick wall.

Islamic State terror group has claimed responsibility for Friday’s incident. The militant group’s Amaq news agency has claimed, via its propoganda media channels, that the explosion was carried out by an affiliated unit.

Mark Rowley, head of counterterrorism for the Metropolitan Police, said police had confirmed the blast was caused by the “detonation of an improvised explosive device”.

He confirmed officers were sifting through surveillance footage and examining the remains of the device.

“This is a very complex investigation which is continuing at speed.

“We are chasing down suspects,” he said.

US President Donald Trump said he spoke to Ms May on Friday (local time) and reinforced America’s commitment to “eradicating the terrorists from our planet”.

Our hearts & prayers go out to the people of London, who suffered a vicious terrorist attack…. pic.twitter.com/Q2hTBV2l5t

— Donald J. Trump (@realDonaldTrump) September 15, 2017

Eyewitness reports of commuters trampled

Fitness instructor Olaniyi Shokunbi was onboard the train when the device exploded.

He said it was fortunate the train was still at the station when the device detonated, instead of travelling through a tunnel.

“We are lucky, because if that train was moving, there is no escaping,” he told the ABC.

Initially, he did not realise what had happened.

“I thought it might be an acid attack,” he said.

Mr Shokunbi said he saw passengers getting crushed by other commuters who were trying to escape.

“Those people had serious injuries, I saw people on the floor, they kind of just crawled out of the station. Possibly broken bones, legs just hanging. You see this stuff in movie … I just can’t believe it.”

Richard Aylmer-Hall, 53, was sitting on the train when panic unfolded.

“Suddenly the whole world charged past me … suddenly there was panic, lots of people shouting, screaming, lots of screaming,” he said.

“There was a woman on the platform who said she had seen a bag, a flash and a bang, so obviously something had gone off.

“Some people got pushed over and trampled on, I saw two women being treated by ambulance crews.

Terror attacks on the rise

Britain has already suffered four terrorist attacks so far this year at a cost of 36 lives.

In 2005, 52 people were killed when four British Islamists carried out suicide bomb attacks on three London underground trains and a bus and this year Britain has suffered four attacks blamed on terrorists.

Television footage showed passengers being escorted off a carriage while a Reuters witness saw armed police scouring a stationary train and a bomb disposal unit at the scene.

In March this year, a man drove a car into pedestrians on London’s Westminster Bridge killing four, before he stabbed a policeman to death outside parliament.

A further 23 people were killed (including the attacker) in a suicide bombing at the Ariana Grande concert at Manchester Arena in May and the following month three Islamist militants drove into pedestrians on London Bridge before stabbing people at nearby restaurants and bars, killing eight.

In June, a van was driven into worshippers near a mosque in north London which left one man dead.

The threat level was last raised to ‘critical’ following the Manchester Arena bombing.

—with ABC/AAP