At least 29 people were injured after the improvised bomb exploded on the rush-hour train in what police say was the fifth terrorist attack in Britain this year.

May said in a televised statement that armed police and members of the military would be seen on the streets in the coming days.

"For this period, military personnel will replace police officers on guard duties at certain protected sites that are not accessible to the public," she said.

Passengers on board the train heading into the capital fled as fire engulfed a carriage at Parsons Green underground station in West London after the explosion at 8.20am local time (5.20pm AEST).

Some suffered burns while others were injured in a stampede to escape. The National Health Service said the injured had been taken to various London hospitals. None were thought to be in a serious condition, the ambulance service said.

"We now assess that this was a detonation of an improvised explosive device," Britain's top counter-terrorism officer Assistant Commissioner Mark Rowley told reporters.

He said most of the injuries were thought to be flash burns.

Assist. Commissioner Rowley declined to answer whether the authorities knew who was responsible or if the suspected bomber had been on the train.

An earlier tweet by US President Donald Trump appeared to suggest those responsible were known to London's Metropolitan Police Service. However Scotland Yard issued a statement saying the President's comments were "pure speculation, given we don't know who's involved. Any speculation is unhelpful".

Pictures taken at the scene showed a white bucket with a supermarket freezer bag on the floor of one train carriage. The bucket was in flames and there appeared to be wires coming out of the top.

British Prime Minister Theresa May confirmed the explosion was being treated as a terrorist attack.

​"Clearly this was a device which was intended to cause significant harm," she said.

"My thoughts are with those injured at Parsons Green and emergency services who are responding bravely to this terrorist incident."

RELATED: Live coverage: Reports of an explosion at Parsons Green

The threat level in the UK was last raised to 'critical' following the Manchester Arena bombing in May, when 23 people, including the attacker, were killed as they exited a concert by US popstar Ariana Grande.

"The public should go about their daily lives but remain vigilant," Mrs May said. "The threat of terrorism we face is severe but by working together we will defeat them."

Asked about Mr Trump's comment that the attack was committed by "people who were in the sights of Scotland Yard", Mrs May repeated the official line from the Metropolitan Police: "I never think it's helpful for anyone to speculate on an ongoing situation," she said.

The Prime Minister chaired a meeting of the government's Cobra emergency committee on Friday afternoon.

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."

RELATED: At least 22 people injured by London Tube 'bucket bomb'

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 seven 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 was urgently trying to find out if any other Australians had 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.

Parsons Green station was closed and paramedics arrived at the scene in under five minutes. Fire crews from Fulham, Wandsworth, Chelsea, Hammersmith and other surrounding fire stations  attended.

Armed police were patrolling at the station, and all train services between Earl's Court and Wimbledon were cancelled. Police were advising people to avoid the area.

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 shocked and tearful and screaming."

Another woman told the reporter she had been on the train, heard a bang and then saw flames. People in front of her shielded her from the flames, she said.

At least one person was taken by stretcher to 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".

With AAP