- Explosion reported at Parsons Green station in west London
- Fire department, British Transport Police, Scotland Yard at the spot
- Images showed a burning bucket stashed in a carrier bag
London Ambulance Service tweeted that 18 people have been taken to a number of hospitals in the city. "We have taken 18 patients to a number of London hospitals. None are thought to be in a serious or life-threatening condition #ParsonsGreen," the tweet said.
The explosion took place inside a plastic bucket left in a supermarket bag on a District Line train when it was at the Parsons Green station in west London.
Witnesses reported seeing passengers who had suffered facial burns and had hair coming off, with at least two women seen being treated by medics amid scenes of panic. On social media, people shared that there was "mass screaming, shouting, stampede and running".
Images from the train showed a burning bucket after the blast which was reported to have sent a "fireball" down the carriage. The bucket looked like the type used by builders and there appeared to be cables coming out of it.
People were seen in tears and some appeared to be in shock as they were led away by policemen.
The fire department, the British Transport police and the Scotland Yard have taken over the area and cordoned it off. The police advised people to stay away.
"We have a number of resources and specialists officers in attendance at the incident at#ParsonsGreen tube station," tweeted the Fire brigade.
One man was quoted as saying he had seen a passenger covered in blood.
This year, Britain has already suffered three terror attacks, in which 36 people have been killed.