A heat wave stifling Brazil set new records on Sunday, with Rio de Janeiro's heat index hitting 62.3 degrees Celsius, the highest in a decade, weather authorities said.

The heat index measures what a temperature feels like, taking into account humidity. The actual maximum temperature in the city was 42 ºC on Monday, the Alerta Rio weather system said.

The 62.3C record was notched in western Rio at 09H55 local time, and was the "highest mark" since Alerta Rio began keeping such records in 2014.

The iconic Ipanema and Copacabana beaches were packed with people as authorities published tips on coping with the heat.

"I am very afraid it will get worse, because the population is increasing a lot and deforestation is very high due to the increase in housing," lamented 49-year-old administrative assistant Raquel Correia in a park in central Rio.

The previous heat index record was set in November, when it hit 59.7C.

Meanwhile, extreme rains were wreaking havoc in the south of the country and are forecast to continue next week, according to authorities.

"The week will be of very high risk in the center-south of Brazil due to intense rains and storms. The most worrisome system is a very intense cold front that will arrive with torrential rains and possible gales," weather information agency MetSul warned Sunday.