Stabbing attack at London tube station

This still from a smartphone video shows the man being arrested at Leytonstone Underground Station in North East London. Photo / Supplied

A man reportedly screamed "this is for Syria" before allegedly slashing a person's throat and attacking up to three people.

Police were called to Leytonstone station after reports of a stabbing in the ticket hall this morning NZT.

One person has been arrested after being tasered by officers at the scene.

AP reports Britain's Scotland Yard says the incident is being treated as "terrorist incident."

The London police counterterror command says in a statement that it is investigating the incident at Leytonstone Underground station in east London.

Authorities were called to the station at around 8am (NZT) to investigate reports of "a number of people" stabbed. One person sustained serious injuries.

A video apparently of the incident and posted on Twitter shows a large pool of blood spattered across the ticket hall as the alleged suspect is tasered by a police officer.

Terrified passengers, some with children, can be seen running across the east London Tube station away from the scene.

In the video the man approaches and gesticulates at people in the station.

As they approach him he lashes out with his knife and appears to slash one of them in the neck area, it is reported.

Moments later a uniformed police officer arrives on the scene and fires his taser at the man.

The terror threat level in Britain remains at severe.

In the video a man approaches and gesticulates at people in the station. Photo / Supplied

- nzherald.co.nz