Laura Kenny crashed out of the opening race of the omnium in a blow to her hopes of a sixth Olympic gold medal.
Kenny was one of a large number of riders to hit the deck in two separate incidents towards the end of the scratch race.
Daria Pikulik of Poland appeared to be the first rider to go down, crashing into Elisa Balsamo and knocking the Italian over.
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That caused further falls a tightly-packed group of riders followed behind them, with Lotte Kopecky of Belgium another of those who tumbled.
"We've just seen that huge crash in the first race of the women's omnium," said Eurosport and discovery+ expert Bradley Wiggins, trackside at the Izu Velodrome.
"Lots of them went down and are now getting bandaged up and looked at by the medics.
Interestingly, Laura hasn't got on the rollers yet. She looked quite angry, quite frustrated with things but she didn't seem too bad physically.
"Incidentally, one of the officials trackside got stretchered away.
"With half an hour until the next race this is crucial timing. Not many of them are warming down yet - we'll see how many starters we get back."
Fortunately, all riders appeared to escape relatively unscathed ahead of the remaining three races of the omnium.

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The crash meant it was a small group that contested the sprint for the scratch race.
It was won by Jennifer Valente of the USA, who pipped reigning world champion Yumi Kajihara.
Australia's Annette Edmonson also managed to stay upright to take third place.
It was the second crash of the race, with Hong Kong's Yao Pang and Olivija Baleisyte of Lithuania colliding with one another about a lap before the bigger group.
Kenny was one of nine riders who failed to finish and were awarded 16 points.
The riders will next compete in the tempo race.
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