A man stabbed at least 10 passengers on a commuter train in Tokyo, just five miles from the Olympic Stadium.
A passenger was seriously injured in the incident, but the suspect was captured by police after fleeing the scene, officials said.
Tokyo firefighters say nine of the victims were taken for treatment at nearby hospitals, while one person was able to escape.
Authorities say all victims remained conscious throughout the incident.
A witness from a nearby station where the train finally stopped, said panicked passengers got out of the cars when it stopped.
NHK public television said a witness told them the passengers were stained with blood and an announcer called the medics on board and all passengers with towels to help them.
The TV station added that the suspect was caught after walking into a convenience store and telling people inside that he was tired of running away.
The store manager saw that the man, who is believed to be in his 20s, was covered in blood and called the police.
The stabbing took place near Seijogakuen station, according to train operator Odakyu Electric Railway Co.
The Tokyo 2020 games are set to end on Sunday after 16 days of competition staged at empty venues and stadiums due to the Covid pandemic.
Police have so far declined to comment on what exactly happened.
Although Japan generally has a low crime rate, it has seen a series of knife attacks with high casualties in recent years.
In May 2019, a 51-year-old man engaged in a wave of stabbing in the town of Kawasaki, in which he killed a 39-year-old man and a 12-year-old schoolgirl, and injured 15 others schoolchildren as they waited for a bus.
And in 2016, 19 residents of a mentally disabled care home were stabbed to death by a former employee who confessed to the murder.
The Associated Press contributed to this report