British Transport Police said on X that officers are "currently responding to an incident on a train to Huntingdon where multiple people have been stabbed".
Cambridgeshire Constabulary, the local police force, said armed police attended after officers were called to Huntingdon station at 7:39pm on Saturday (6:39am AEDT on Sunday).
Huntingdon station is around 120km north of London.
"Officers are in attendance alongside @CambsCops and two people have been arrested."
Witnesses described seeing a man with a large knife and passengers hiding in the toilets to escape the rampage, The Times reported.
One witness told the paper there was "blood everywhere" and people were getting "stamped" on by others as they tried to flee.
"I heard some people shouting we love (you)," the witness said.
Video footage on social media showed scores of blue-light police cars and emergency vehicles in a station car park, and a team of armed police running towards the stationary train at Huntingdon station.
An eyewitness said the scene on board the train was "like something from a film", the Sun reported.
"There was blood everywhere, it was a terrible scene, really violent. It was like something from a film," the witness told the paper.
"It didn't feel real and just sparked pure panic on board.
"Shocked onlookers were shaken and traumatised."
The attack is understood to have started shortly after the LNER train left Peterborough station.
In a statement on Facebook, Cambridgeshire Constabulary said: "We were called at 7.39pm with reports that multiple people had been stabbed on a train.
"Armed officers attended and the train was stopped at Huntingdon, where two men were arrested. A number of people have been taken to hospital.
"The incident remains ongoing and the A1307 has been closed as you approach the town centre."
Huntingdon Station is closed, National Rail said, with all lines blocked.
Prime Minister Keir Starmer said the incident was "deeply concerning" and urged people to follow police advice.
"The appalling incident on a train near Huntingdon is deeply concerning," he said on X.
"My thoughts are with all those affected, and my thanks go to the emergency services for their response.
"Anyone in the area should follow the advice of the police."
Paul Bristow, the mayor of Cambridgeshire and Peterborough, said he had heard of "horrendous scenes" on the train.
London North Eastern Railway, which operates East Coast Mainline services in the UK, has urged passengers not to travel because of "major disruption".
In a statement on X at 9.21pm, LNER said: "We are experiencing major disruption across the LNER route. Emergency services are dealing with an incident at Huntingdon station, all lines are blocked.
"Our advice is 'Do Not Travel'. Please defer your travel where you can."
with AP
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