Nearly 300 swimmers looking to cool off from a heat wave were stung by swarms of jellyfish that filled the shallow waters off a southeastern beach city, Brazilian media reported Sunday.

At least 15 people including children and teenagers were treated in Praia Grande for severe stings, doctor Adriano Bechara told the Tribuna newspaper, though their lives were not in danger.

Many of the injured arrived at medical centers Friday and Saturday crying from the pain of the stings, paramedic Claudio Casadei told Tribuna. Most were treated and released.

Authorities blamed an extreme heat wave over the southeastern region for the swarm.

Hundreds of thousands of Brazilians flock to the country's beaches every year during the holiday season.