National Health Service says 22 people were taken to London hospitals.

An explosion on a London Underground train on Friday morning is being treated as a terrorist incident, British police said.

The Metropolitan Police said emergency services were called to Parsons Green station in west London at 8.20 a.m. local time (0720GMT) following reports of a fire on a commuter train.

Londra metrosunda patlama: Yaralılar var ile ilgili görsel sonucu

A “number of people suffered injuries” in the blast, police said and counterterrorist officers have declared it a terrorist incident.

The National Health Service (NHS) said on its Twitter account that 22 patients were taken to London hospitals. Earlier, the London Ambulance Service had said 18 people were being treated for non-life-threatening injuries.

Britain's most senior counterterrorism officer Mark Rowley said in a press conference that most of the injured had received "flash burns".  Rowley said the explosion was caused by a suspected "Improvised Explosive Device" (IED).

He said more police would be deployed across London especially on the transport network.

Online /images showed a white bucket with wires protruding from it on fire inside a supermarket bag.

'Calm and vigilant'

Prime Minister Theresa May will attend an emergency COBRA (Cabinet Office Briefing Room A -- where crisis response meetings generally take place) meeting later Friday.

“My thoughts are with those injured at Parsons Green and the emergency services who, once again, are responding swiftly and bravely to a suspected terrorist incident,” she said in a statement.

London Mayor Sadiq Khan said the city “utterly condemns the hideous individuals who attempt to use terror to harm us and destroy our way of life”.

He urged Londoners to “remain calm and vigilant”.

U.S President Donald Trump, who had criticized the London mayor following another attack in the British capital in early June, also reacted to the latest incident.

"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 wrote on Twitter.

"Loser terrorists must be dealt with in a much tougher manner. The internet is their main recruitment tool which we must cut off & use better!"

Londra metrosunda patlama: Yaralılar var ile ilgili görsel sonucu

High alert

Parsons Green station and the roads around it were cordoned off and police advised people to avoid the area.

Footage from the scene also showed passengers being led off a train by police.

The ambulance service said it had sent a special hazardous area response team to the scene, amid reports of a white substance being found.

Britain has been on high alert following four terror attacks in London and Manchester this year which led to 36 deaths.

Around 5 million people use London's metro system daily.

In 2007, 52 people were killed in underground train and bus explosions in a series of coordinated suicide bomb attacks in London.

Londra metrosunda patlama: Yaralılar var ile ilgili görsel sonucu 
 Londra metrosunda patlama: Yaralılar var ile ilgili görsel sonucu
 Londra metrosunda patlama: Yaralılar var ile ilgili görsel sonucu