A sudden suspension of the sole train to Japan's Expo 2025 stranded more than 30,000 visitors, with some forced to spend a sweltering night near the station and more than 30 people sent to hospitals by Thursday morning.

A power outage abruptly shut the metro line in Osaka on Wednesday night while a crowd was packed into the expo site's station.

As of 9.30pm on Wednesday, around 30,000 people were stranded, the expo organiser said.

"So many people were packing the station that it felt like a steam bath," an elderly woman told local broadcaster MBS News.

Many, including families with children, wound up sleeping on benches under the Expo's wooden "Grand Ring" or inside pavillons turned into shelters.

It took the Osaka Metro around eight hours to restore the Chuo line's service, with trains not fully operational until early on Thursday.

By then, 36 people had been sent to hospitals for symptoms including headaches and dizziness, according to the expo organiser.

Expo staffers spent the night trying to cheer trapped visitors by lighting up objects and turning on music.

Bowing deeply, Osaka Metro officials apologised for "causing immense trouble" to those affected, blaming a short-circuit for the outage.

Japan has been suffering a punishing summer heatwave, recording its hottest July since records began in 1898.

Osaka last hosted the expo in 1970 when it attracted 64 million people, a record until Shanghai in 2010.

Last month, expo organisers reported welcoming 10 million visitors, with the fair running until October. (AFP)