The heat index, which combines temperature and humidity, reached 59.7C (139.5F) in the Brazilian city on Saturday.

In addition, a fan died before Swift's show on Friday, leaving her "devastated".

Ana Clara Benevides, 23, became unwell and later died in hospital. Her cause of death has not been announced but Brazilian daily newspaper Fohla de S Paulo reported she had collapsed at the show and had a cardiac arrest.

Several fans fell ill on Friday, prompting Swift to interrupt her performance and ask the production team to give them water, videos showed.

The concerts are being held at Nilton Santos stadium, far from Rio's breezy beaches in a hotter northern neighbourhood.

Swift posted the decision to postpone Saturday's show on Instagram about two hours before it was due to start.

"The safety and well-being of my fans, fellow performers, and crew has to, and always will, come first," she said.

As news of the postponement spread, many young girls outside the stadium burst into tears.

Swift is also due to perform in Rio on Sunday, ahead of three shows in Sao Paulo next weekend.

Heat exhaustion, which can include dizziness, headaches, shaking and thirst, is not usually serious, provided the person cools down within 30 minutes.

However, heat stroke - when the body's core temperature goes above 40.6C - is a medical emergency.

Following Ms Benevides' death, justice minister Flavio Dino said the Brazilian government would issue a directive allowing water bottles to be brought into concerts.

"Companies producing shows with high heat exposure must provide free drinking water in easily accessible 'hydration islands'," he said on X, formerly known as Twitter.

"The measure is effective immediately."