Image copyright PA
London Underground passengers have been hurt after an explosion on a District Line train in south-west London.
Police and paramedics were called at 08:20 BST (07:20 GMT) on Friday to Parsons Green station in Fulham.
Pictures show a white bucket on fire inside a supermarket bag, but do not appear to show extensive damage to the inside of the Tube train carriage.
BBC Security Correspondent Frank Gardner said the Met Counter Terrorism Command is leading the response.
It is too early to say who caused the explosion, he added.
Witnesses described seeing at least one passenger with facial injuries.
Others have spoken of "panic" as alarmed passengers left the train at Parsons Green station.
London Ambulance Service says it has sent a hazardous area response team to the scene.
Media playback is unsupported on your device Media caption Witnesses describe tube 'explosion' at Parsons Green
Passenger Chris Wildish told BBC Radio 5 live he saw a bucket in a supermarket bag with "low-level flames coming out of it" by the door of the rear carriage.
One witness, called Luke, told 5 live there was "a sort of loud explosion".
"It happened just as we were pulling up to the Tube station so everyone just sort of piled out of the Tube and there was a distinct smell of burning," he said.
"I certainly saw some burning injuries," he said, but added that "everyone behaved in the right manner as such, everyone got off as quickly as they can and you know supported everyone".
Media playback is unsupported on your device Media caption BBC presenter Sophie Raworth was near Parsons Green station minutes after the incident happened
Emma, who was at Parsons Green station, said: "We were running down the stairs like... it felt like for our lives.
"I went down the stairs and after a while people were just piling on top of each other, because people were falling over trying to run so quickly.
"There were two ladies underneath me and a little boy to my right, his head had been smacked into the concrete."
Media playback is unsupported on your device Media caption One commuter told of a "wall of flame" that came down the carriage after the explosion
BBC London presenter Riz Lateef, who was at Parsons Green on her way in to work, said: "There was panic as people rushed from the train, hearing what appeared to be an explosion.
"People were left with cuts and grazes from trying to flee the scene. There was lots of panic."
Image copyright AFP
BBC News presenter Sophie Raworth says she saw a woman on a stretcher with burns to her face and legs.
Alex Littlefield, 24, a City worker, said: "I was walking around the corner to the Parsons Green Tube station and I saw the raised platform with everyone running and looking upset.
"I saw police officers, fire brigade... masses of people and armed police. There were lots of very, very distressed people. We've been pushed right back now."
Image copyright Reuters
Image copyright Reuters
Media technology consultant Richard Aylmer-Hall who was sitting on the "packed" District Line train said he saw several people injured, having apparently been trampled as they tried to escape.
The 53-year-old said: "Suddenly there was panic, lots of people shouting, screaming, lots of screaming.
"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.
"I saw crying women, there was lots of shouting and screaming, there was a bit of a crush on the stairs going down to the streets."
Image copyright @emmastevie1
Natasha Wills, assistant director of operations at London Ambulance Service, said: "We were called at 8:20 to reports of an incident at Parsons Green underground station.
"We have sent multiple resources to the scene including single responders in cars, ambulance crews, incident response officers and our hazardous area response team, with the first of our medics arriving in under five minutes.
"Our initial priority is to assess the level and nature of injuries. More information will follow when we have it."
Image copyright Alex Littlefield
Are you at Parsons Green station? Did you witness the events? If it's safe to share your experiences then please email haveyoursay@bbc.co.uk with your stories.
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Symbolbild: Ein Zug im Londoner Untergrund
 In der britischen Hauptstadt London kam es am Morgen des 15. Septembers um etwa 8 Uhr 20 zur Explosion einer Bombe in der Metro. Der Vorfall wurde als Terroranschlag eingestuft. Dreißig Personen erlitten Verletzungen, ein Großteil davon Verbrennungen.
Polizei am Freitag nahe dem Tatort
Die Polizei erhöhte nach dem Anschlag die Terrorwarnstufe im Land auf die Stufe fünf „kritisch“, der höchsten Stufe der Skala. Nach der Festnahme eines 18-jährigen Mannes am Hafen von Dover wurde die Warnstufe auf vier „ernst“ heruntergesetzt. Nach Auskunft von Scotland Yard soll es sich bei dem Festgenommenen um die Personen handeln, von der die Bombe im Zug platziert worden war.

Am Sonntag nahm die Polizei eine 21-jährigen Mann im Westen von London im Stadtteil Hounslow fest. Im Zusammenhang mit der Festnahme durchsuchten Ermittler eine Wohnung nahe dem Flughafen Heathrow in der Ortschaft Stanwell.