London's Metropolitan police, counter terror police and emergency services are attending to an incident at a train station west of the city following reports of an explosion on a train.
Authorities responded to reports of an explosion on a packed peak hour train at Parsons Green station, leaving several passengers injured.
Photos circulating on social media show a white bucket alight inside a large plastic bag.
News services are reporting some people suffered facial burns as the container "blew up" towards the rear of the train about 8.20am local time (5.20pm AEST).
Police confirmed they were treating it as a terror attack, saying: "The Met's Counter Terrorism Command are investigating after the incident at Parsons Green tube station is declared a terrorist incident", but added it was "too early to confirm cause of fire" and are asking witnesses to upload any photos of the scene to a secure website which will form part of the investigation.
London Ambulance confirmed at least 18 people were injured, none are thought to be in a serious or life-threatening condition and being treated at various hospitals around the city. Another four later presented themselves at hospitals with non-life threatening injuries.
Australian Hayden Locke was on board the "packed" train at the time of the explosion.
The 35-year-old father-of-two who has been living in London for the past three years was heading into London for meetings when people started stampeding from the back of the train as the train pulled into Parsons Green station.
"People kept yelling there was a bomb. People are really on edge here," he told Fairfax Media.
"All I could see was a little smoke, I just thought it was an electrical fire but people just started running. More people have been hurt by the rush."
Locke, who works in the mining industry, said hundreds of people fled the train with many being injured trying to flee the train and the underground station.
"I saw one woman whose hair was all burnt and another woman had hurt her back after being caught up trying to get out of the station," he said.
Due to traffic and public transport disruptions he now has to ride a bike the remaining 7-kilometres into London to start his work day. His wife and young children came to the station to meet him after the incident.
"My kids are still too young to ride the tube, thank god," he said.
The federal government is urgently trying to find out if any other Australians have been caught up in the explosion.
Foreign Minister Julie Bishop has been in contact with UK High Commissioner Alexander Downer, a Department of Foreign Affairs and Trade spokeswoman said.
"The Australian High Commission in London is making urgent inquiries to determine whether any Australians have been affected by an explosion aboard a commuter train at Parsons Green Station, London," she said.
In a short statement British Prime Minister Theresa May said: "My thoughts are with those injured at Parsons Green and emergency services who are responding bravely to this terrorist incident".
The PM will chair a meeting of the government's Cobra emergency committee on Friday afternoon.
Parsons Green station has been closed and paramedics arrived at the scene in under five minutes. Fire crews from Fulham, Wandsworth, Chelsea, Hammersmith and other surrounding fire stations are in attendance.
Armed police are at the station. All train services between Earls Court and Wimbledon have been cancelled. Police, via social media, are advising people to "avoid the area".
"We have sent multiple resources to the scene including single responders in cars, ambulance crews, incident response officers and our hazardous area response team," London Ambulance Service's assistant director of operations Natasha Wills said.
"Our initial priority is to assess the level and nature of injuries."
BBC reporter Riz Lateef was at Parsons Green, and described the panic as people tried to flee.
"There was panic as people rushed from the train, hearing what appeared to be an explosion," she said.
"People were left with cuts and grazes from trying to flee the scene"
"There was lots of panic."
A witness told the BBC he was coming towards the station then "suddenly everyone came running down the stairs...lots of people shoked and tearful and screaming."
Another woman told the reporter she had been on the train, heard a bang then saw flames. There were people in front of her that shielded her from the flames.
At least one person was stretchered into an ambulance. She was conscious but her legs and face were burnt according to reports.
The carriages would have been "like sardines" at that time of rush hour, regular commuters on the line said.
Another witness told the BBC he saw scenes of desperate panic as people fled the scene.
There were "hordes of people trying to get out as quickly as they could", he said. "People were falling and tumbling on top of each other...crying and really shaken up".
Reports of explosion on London Tube at Parsons Green station
EARLIER: London's Metropolitan police and emergency services are attending to an incident at a train station west of the city following reports of an explosion on a train.
London's The Telegraph is reporting there was an explosion on a train at Parsons Green station, leaving several passengers injured.
Photos circulating on social media show a white bucket alight inside a large plastic bag.
News services are reporting some people suffered facial burns as the container "blew up" towards the rear of the train during busy rush hour about 8.20am local time (5.20pm AEST).
One witness reported seeing "a flash and a bang" in a bag, The Telegraph reported.
The Parsons Green station has been closed and paramedics arrived at the scene in under five minutes.
Armed police are at the station. All train services between Earls Court and Wimbledon have been cancelled. Police, via social media, are advising people to "avoid the area".
"We have sent multiple resources to the scene including single responders in cars, ambulance crews, incident response officers and our hazardous area response team," London Ambulance Service's assistant director of operations Natasha Wills said.
"Our initial priority is to assess the level and nature of injuries."
BBC reporter Riz Lateef was at Parsons Green, and described the panic as people tried to flee.
"There was panic as people rushed from the train, hearing what appeared to be an explosion," she said.
"People were left with cuts and grazes from trying to flee the scene"
"There was lots of panic."
