A homemade bomb that only partially detonated on a packed London subway train injured 22 people but left no one seriously hurt, police say.

Commuters reported a noise and a flash aboard the District Line train at Parsons Green station in the city’s south-west of the city, followed by chaos as hundreds of people rushed to get away from danger.

“It was this flame of orange coming towards you,” recalled a passenger who asked to be identified only as “Lauren”. “We just panicked … and we ran in the opposite direction”

Mark Rowley, head of counterterrorism for the Metropolitan Police, said police had confirmed the blast was caused by the “detonation of an improvised explosive device”.

He said 18 people had been taken to hospital by ambulance, mostly with “flash burns”. Another four people presented themselves to hospitals, health officials said.

“I was on the second carriage from the back. I just heard a kind of whoosh. I looked up and saw the whole carriage engulfed in flames making its way towards me,” said passenger Ola Fayankinnu.

“A lot of people were trampled on. There were phones, hats, bags all over the place, and when I looked back I saw a bag with flames.

People were crying, shocked, a few people had been injured, some people had been trampled.”

The bomb, which authorities say only partially detonated, exploded shortly after 8am as the train loaded with commuters on their way to work pulled into the platform and opened its doors.

The line between Earl’s Court and Wimbledon was immediately shut down as firemen, anti-terrorism police and the bomb squad flooded the station and surrounding streets.

An injured woman is assisted from the chaos at Parsons Green station, where a bomb sent a “wall of flame” through the crowded carriage.

Prime Minister Theresa May would chair an immediate crisis meeting of Britain’s emergency response committee, her office said.

“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,” May said.

US President Donald Trump has also weighed in on Twitter, sending out a series of tweets.

“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.

An unverified picture circulating on social media showed a white bucket in a supermarket freezer bag with what appeared to be wires coming out of the top on the floor of one train carriage.

Richard Aylmer-Hall, 53, was sitting on the train when panic unfolded.

“Suddenly the whole world charged past me … suddenly there was panic, lots of people shouting, screaming, lots of screaming,” he said.

“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.

“Some people got pushed over and trampled on, I saw two women being treated by ambulance crews.

“I don’t think anyone was hurt by the actual device or whatever it was.”

A lone policeman unpacks his gear outside the carriage where commuters fled in terror.

Britain has already suffered four terrorist attacks so far this year at a cost of 36 lives.

In 2005, 52 people were killed when four British Islamists carried out suicide bomb attacks on three London underground trains and a bus and this year Britain has suffered four attacks blamed on terrorists.

Television footage showed passengers being escorted off a carriage while a Reuters witness saw armed police scouring a stationary train and a bomb disposal unit at the scene.

In March this year, a man drove a car into pedestrians on London’s Westminster Bridge killing four, before he stabbed a policeman to death outside parliament.

A further 22 people were killed in a suicide bombing at a pop concert in Manchester in May and the following month three Islamist militants drove into pedestrians on London Bridge before stabbing people at nearby restaurants and bars, killing eight.

In June, a van was driven into worshippers near a mosque in north London which left one man dead.

-with AAP and ABC