Over 18 people, including a schoolboy, were injured in an explosion on a London Tube train during the Friday morning rush hour sparking panic among the passengers.

Police are treating the blast as a terrorist incident after commuters reported a "fireball flying down a carriage" as the bomb, hidden in a bucket in a Lidl bag, partially detonated inside the carriage.

Passengers, including children, fled the District Line train after the explosion, which happened at Parsons Green Station, said media reports.

Eyewitnesses reported seeing some people with facial burns and others with injuries caused by the stampede in the wake of the incident, which happened at around 8.20am on Friday.

Prime Minister Theresa May, who condemned the incident, tweeted: "My thoughts are with those injured at Parsons Green and emergency services who are responding bravely to this terrorist incident."

She is set to chair a meeting of the Government's Cobra emergencies committee this afternoon to discuss the incident, reported the Telegraph.

It comes with Britain on high alert after four terror attacks in 2017. The official threat level currently stands at severe - meaning an attack is seen as "highly likely."

Home Secretary Amber Rudd has condemned the Parsons Green blast, saying:  "Once more people going about their everyday lives have been targeted in a callous and indiscriminate way.

"My thoughts are with all those injured and affected. The police and emergency services were quickly on the scene and I would like to pay tribute again to their professionalism and tireless commitment, " Rudd was quoted as saying in the Telegraph.

"I am receiving regular updates on what is an ongoing and active investigation. I would urge Londoners to remain alert and assist the police and emergency services as much as they can," he added.

Officers from the Metropolitan Police’s Counter Terrorism Command are investigating after the incident.

A statement from the Metropolitan Police said: “At present we are aware of a number of people who have suffered injuries.

“It is too early to confirm the cause of the fire, which will be subject to the investigation that is now underway by the Met’s Counter Terrorism Command.

“The station remains cordoned off and we are advising people to avoid the area.”

Eighteen people were hospitalised this morning when an improvised bomb exploded on a District Line Underground train at Parsons Green.

Police have announced they are treating the incident as a terror attack.

The London terror attack suspect is believed to have fled the scene.

Amid this chaos, photos shared on social media showed armed police surrounding the red Transport for London bus on Chelsea Bridge, reported Express.

One Twitter user said: "Looks like a bus has been stopped by armed police near Chelsea bridge.”

Another said: “Armoured police vehicle and 4x4s surround a bus on Chelsea Bridge Road. Armed police everywhere.”

Meanwhile, Mayor of London Sadiq Khan has appealed for calm, saying the city "will never be intimidated or defeated by terrorism".

Witnesses have described seeing at least one passenger with facial injuries. Others have spoken of "panic" as alarmed passengers left the train at Parsons Green station, reported BBC.

Reports via social media shared pictures of what was believed to be the device.

Some pictures showed what looked like a white container with flames flickering inside, while other people said they had seen a bag with wires on the train.