Violent crime is rare in Japan, and the attack comes with the capital on heightened security alert as it hosts the Olympic Games.

The stabbings took place on an evening train in Setagaya ward, in the west of the city.

One woman in her 20s sustained serious back injuries, public broadcaster NHK said, citing emergency response officials.

The site is several kilometres from the venue of Olympic equestrian events.

The train was brought to an emergency stop after the rail operator was alerted, and a knife and mobile phone believed to belong to the suspect were found onboard.

The male suspect, also in his 20s, was reportedly taken in by police after he turned himself in at a nearby convenience store, telling the manager he was the perpetrator of the attack.

Japan has strict gun laws, but there are occasional violent crimes involving other weapons.

In 2019, a man killed two people including a schoolgirl and wounded more than a dozen in a rampage that targeted children as they waited for a bus.

And in 2018, a man was arrested in central Japan after stabbing one person to death and injuring two others aboard a bullet train.